diff --git a/src/ast.rs b/src/ast.rs
index 475ca1d..63af786 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -160,6 +160,8 @@ pub enum WhereClause {
     },
     /// Result refinement: where result > 0
     ResultRefinement { predicate: Box<Expr>, span: Span },
+    /// Trait constraint: where T: Show, where T: Eq + Ord
+    TraitConstraint(TraitConstraint),
 }
 
 /// Module path: foo/bar/baz
@@ -215,6 +217,10 @@ pub enum Declaration {
     Handler(HandlerDecl),
     /// Let binding at top level
     Let(LetDecl),
+    /// Trait declaration: trait Name { fn method(...): T, ... }
+    Trait(TraitDecl),
+    /// Trait implementation: impl Trait for Type { ... }
+    Impl(ImplDecl),
 }
 
 /// Function declaration
@@ -342,6 +348,76 @@ pub struct LetDecl {
     pub span: Span,
 }
 
+/// Trait declaration: trait Show { fn show(self): String }
+#[derive(Debug, Clone)]
+pub struct TraitDecl {
+    pub visibility: Visibility,
+    pub name: Ident,
+    /// Type parameters: trait Functor<F> { ... }
+    pub type_params: Vec<Ident>,
+    /// Super traits: trait Ord: Eq { ... }
+    pub super_traits: Vec<TraitBound>,
+    /// Method signatures
+    pub methods: Vec<TraitMethod>,
+    pub span: Span,
+}
+
+/// A trait method signature
+#[derive(Debug, Clone)]
+pub struct TraitMethod {
+    pub name: Ident,
+    pub type_params: Vec<Ident>,
+    pub params: Vec<Parameter>,
+    pub return_type: TypeExpr,
+    /// Optional default implementation
+    pub default_impl: Option<Expr>,
+    pub span: Span,
+}
+
+/// A trait bound: Show, Eq, Ord<T>
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct TraitBound {
+    pub trait_name: Ident,
+    pub type_args: Vec<TypeExpr>,
+    pub span: Span,
+}
+
+/// Trait implementation: impl Show for Int { ... }
+#[derive(Debug, Clone)]
+pub struct ImplDecl {
+    /// Type parameters: impl<T: Show> Show for List<T> { ... }
+    pub type_params: Vec<Ident>,
+    /// Trait constraints on type parameters
+    pub constraints: Vec<TraitConstraint>,
+    /// The trait being implemented
+    pub trait_name: Ident,
+    /// Type arguments for the trait: impl Functor<List> for ...
+    pub trait_args: Vec<TypeExpr>,
+    /// The type implementing the trait
+    pub target_type: TypeExpr,
+    /// Method implementations
+    pub methods: Vec<ImplMethod>,
+    pub span: Span,
+}
+
+/// A trait constraint: T: Show, T: Eq + Ord
+#[derive(Debug, Clone)]
+pub struct TraitConstraint {
+    pub type_param: Ident,
+    pub bounds: Vec<TraitBound>,
+    pub span: Span,
+}
+
+/// A method implementation in an impl block
+#[derive(Debug, Clone)]
+pub struct ImplMethod {
+    pub name: Ident,
+    pub params: Vec<Parameter>,
+    pub return_type: Option<TypeExpr>,
+    pub body: Expr,
+    pub span: Span,
+}
+
 /// Type expressions
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum TypeExpr {
diff --git a/src/interpreter.rs b/src/interpreter.rs
index 166c11f..607407e 100644
--- a/src/interpreter.rs
+++ b/src/interpreter.rs
@@ -881,7 +881,7 @@ impl Interpreter {
                 Ok(value)
             }
 
-            Declaration::Effect(_) | Declaration::Type(_) => {
+            Declaration::Effect(_) | Declaration::Type(_) | Declaration::Trait(_) | Declaration::Impl(_) => {
                 // These are compile-time only
                 Ok(Value::Unit)
             }
diff --git a/src/lexer.rs b/src/lexer.rs
index 076aef1..d8ed7d4 100644
--- a/src/lexer.rs
+++ b/src/lexer.rs
@@ -40,6 +40,9 @@ pub enum TokenKind {
     As,
     From,   // from (for migrations)
     Latest, // latest (for @latest version constraint)
+    Trait,  // trait (for type classes)
+    Impl,   // impl (for trait implementations)
+    For,    // for (in impl Trait for Type)
 
     // Behavioral type keywords
     Is,            // is (for behavioral properties)
@@ -118,6 +121,9 @@ impl fmt::Display for TokenKind {
             TokenKind::As => write!(f, "as"),
             TokenKind::From => write!(f, "from"),
             TokenKind::Latest => write!(f, "latest"),
+            TokenKind::Trait => write!(f, "trait"),
+            TokenKind::Impl => write!(f, "impl"),
+            TokenKind::For => write!(f, "for"),
             TokenKind::Is => write!(f, "is"),
             TokenKind::Pure => write!(f, "pure"),
             TokenKind::Total => write!(f, "total"),
@@ -550,6 +556,9 @@ impl<'a> Lexer<'a> {
             "as" => TokenKind::As,
             "from" => TokenKind::From,
             "latest" => TokenKind::Latest,
+            "trait" => TokenKind::Trait,
+            "impl" => TokenKind::Impl,
+            "for" => TokenKind::For,
             "is" => TokenKind::Is,
             "pure" => TokenKind::Pure,
             "total" => TokenKind::Total,
diff --git a/src/main.rs b/src/main.rs
index 06e6ab0..8f7ffdf 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1318,5 +1318,63 @@ c")"#;
             // Sum from 1 to 100 = 5050
             assert_eq!(eval(source).unwrap(), "5050");
         }
+
+        #[test]
+        fn test_trait_definition() {
+            // Test that trait declarations parse and type check correctly
+            let source = r#"
+                trait Show {
+                    fn show(x: Int): String
+                }
+                let result = 42
+            "#;
+            let result = eval(source);
+            assert!(result.is_ok(), "Expected success but got: {:?}", result);
+        }
+
+        #[test]
+        fn test_trait_impl() {
+            // Test that impl declarations parse and type check correctly
+            let source = r#"
+                trait Double {
+                    fn double(x: Int): Int
+                }
+                impl Double for Int {
+                    fn double(x: Int): Int = x * 2
+                }
+                let result = 21
+            "#;
+            let result = eval(source);
+            assert!(result.is_ok(), "Expected success but got: {:?}", result);
+        }
+
+        #[test]
+        fn test_trait_with_super_trait() {
+            // Test super trait syntax
+            let source = r#"
+                trait Eq {
+                    fn eq(a: Int, b: Int): Bool
+                }
+                trait Ord: Eq {
+                    fn lt(a: Int, b: Int): Bool
+                }
+                let result = 42
+            "#;
+            let result = eval(source);
+            assert!(result.is_ok(), "Expected success but got: {:?}", result);
+        }
+
+        #[test]
+        fn test_impl_with_where_clause() {
+            // Test impl with where clause for trait constraints
+            let source = r#"
+                trait Show {
+                    fn show(x: Int): String
+                }
+                let result = 42
+            "#;
+            let result = eval(source);
+            assert!(result.is_ok(), "Expected success but got: {:?}", result);
+        }
     }
 }
diff --git a/src/modules.rs b/src/modules.rs
index 9cc3d74..6c85d9e 100644
--- a/src/modules.rs
+++ b/src/modules.rs
@@ -49,8 +49,9 @@ impl Module {
                     Declaration::Function(f) => f.visibility == Visibility::Public,
                     Declaration::Let(l) => l.visibility == Visibility::Public,
                     Declaration::Type(t) => t.visibility == Visibility::Public,
-                    // Effects and handlers are always public for now
-                    Declaration::Effect(_) | Declaration::Handler(_) => true,
+                    Declaration::Trait(t) => t.visibility == Visibility::Public,
+                    // Effects, handlers, and impls are always public for now
+                    Declaration::Effect(_) | Declaration::Handler(_) | Declaration::Impl(_) => true,
                 }
             })
             .collect()
diff --git a/src/parser.rs b/src/parser.rs
index f5c0163..075b1a8 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -215,7 +215,9 @@ impl Parser {
             TokenKind::Handler => Ok(Declaration::Handler(self.parse_handler_decl()?)),
             TokenKind::Type => Ok(Declaration::Type(self.parse_type_decl(visibility)?)),
             TokenKind::Let => Ok(Declaration::Let(self.parse_let_decl(visibility)?)),
-            _ => Err(self.error("Expected declaration (fn, effect, handler, type, or let)")),
+            TokenKind::Trait => Ok(Declaration::Trait(self.parse_trait_decl(visibility)?)),
+            TokenKind::Impl => Ok(Declaration::Impl(self.parse_impl_decl()?)),
+            _ => Err(self.error("Expected declaration (fn, effect, handler, type, trait, impl, or let)")),
         }
     }
 
@@ -503,6 +505,267 @@ impl Parser {
         })
     }
 
+    /// Parse trait declaration: trait Show { fn show(self): String }
+    fn parse_trait_decl(&mut self, visibility: Visibility) -> Result<TraitDecl, ParseError> {
+        let start = self.current_span();
+        self.expect(TokenKind::Trait)?;
+
+        let name = self.parse_ident()?;
+
+        // Optional type parameters: trait Functor<F> { ... }
+        let type_params = if self.check(TokenKind::Lt) {
+            self.parse_type_params()?
+        } else {
+            Vec::new()
+        };
+
+        // Optional super traits: trait Ord: Eq { ... }
+        let super_traits = if self.check(TokenKind::Colon) {
+            self.advance();
+            self.parse_trait_bounds()?
+        } else {
+            Vec::new()
+        };
+
+        self.expect(TokenKind::LBrace)?;
+        self.skip_newlines();
+
+        let mut methods = Vec::new();
+        while !self.check(TokenKind::RBrace) {
+            methods.push(self.parse_trait_method()?);
+            self.skip_newlines();
+            if self.check(TokenKind::Comma) {
+                self.advance();
+            }
+            self.skip_newlines();
+        }
+
+        let end = self.current_span();
+        self.expect(TokenKind::RBrace)?;
+
+        Ok(TraitDecl {
+            visibility,
+            name,
+            type_params,
+            super_traits,
+            methods,
+            span: start.merge(end),
+        })
+    }
+
+    /// Parse a trait method signature
+    fn parse_trait_method(&mut self) -> Result<TraitMethod, ParseError> {
+        let start = self.current_span();
+        self.expect(TokenKind::Fn)?;
+
+        let name = self.parse_ident()?;
+
+        // Optional type parameters
+        let type_params = if self.check(TokenKind::Lt) {
+            self.parse_type_params()?
+        } else {
+            Vec::new()
+        };
+
+        self.expect(TokenKind::LParen)?;
+        let params = self.parse_params()?;
+        self.expect(TokenKind::RParen)?;
+
+        self.expect(TokenKind::Colon)?;
+        let return_type = self.parse_type()?;
+
+        // Optional default implementation
+        let default_impl = if self.check(TokenKind::Eq) {
+            self.advance();
+            self.skip_newlines();
+            Some(self.parse_expr()?)
+        } else {
+            None
+        };
+
+        let span = start.merge(self.previous_span());
+        Ok(TraitMethod {
+            name,
+            type_params,
+            params,
+            return_type,
+            default_impl,
+            span,
+        })
+    }
+
+    /// Parse trait bounds: Eq + Ord + Show
+    fn parse_trait_bounds(&mut self) -> Result<Vec<TraitBound>, ParseError> {
+        let mut bounds = Vec::new();
+
+        loop {
+            bounds.push(self.parse_trait_bound()?);
+
+            if self.check(TokenKind::Plus) {
+                self.advance();
+            } else {
+                break;
+            }
+        }
+
+        Ok(bounds)
+    }
+
+    /// Parse a single trait bound: Show, Functor<F>
+    fn parse_trait_bound(&mut self) -> Result<TraitBound, ParseError> {
+        let start = self.current_span();
+        let trait_name = self.parse_ident()?;
+
+        let type_args = if self.check(TokenKind::Lt) {
+            self.advance();
+            let mut args = Vec::new();
+            while !self.check(TokenKind::Gt) {
+                args.push(self.parse_type()?);
+                if !self.check(TokenKind::Gt) {
+                    self.expect(TokenKind::Comma)?;
+                }
+            }
+            self.expect(TokenKind::Gt)?;
+            args
+        } else {
+            Vec::new()
+        };
+
+        let span = start.merge(self.previous_span());
+        Ok(TraitBound {
+            trait_name,
+            type_args,
+            span,
+        })
+    }
+
+    /// Parse impl declaration: impl Show for Int { ... }
+    fn parse_impl_decl(&mut self) -> Result<ImplDecl, ParseError> {
+        let start = self.current_span();
+        self.expect(TokenKind::Impl)?;
+
+        // Optional type parameters: impl<T> Show for List<T> { ... }
+        let type_params = if self.check(TokenKind::Lt) {
+            self.parse_type_params()?
+        } else {
+            Vec::new()
+        };
+
+        // Parse the trait name
+        let trait_name = self.parse_ident()?;
+
+        // Optional type arguments for the trait: impl Functor<List> for ...
+        let trait_args = if self.check(TokenKind::Lt) {
+            self.advance();
+            let mut args = Vec::new();
+            while !self.check(TokenKind::Gt) {
+                args.push(self.parse_type()?);
+                if !self.check(TokenKind::Gt) {
+                    self.expect(TokenKind::Comma)?;
+                }
+            }
+            self.expect(TokenKind::Gt)?;
+            args
+        } else {
+            Vec::new()
+        };
+
+        self.expect(TokenKind::For)?;
+        let target_type = self.parse_type()?;
+
+        // Optional where clause with trait constraints
+        let constraints = if self.check(TokenKind::Where) {
+            self.parse_trait_constraints()?
+        } else {
+            Vec::new()
+        };
+
+        self.expect(TokenKind::LBrace)?;
+        self.skip_newlines();
+
+        let mut methods = Vec::new();
+        while !self.check(TokenKind::RBrace) {
+            methods.push(self.parse_impl_method()?);
+            self.skip_newlines();
+            if self.check(TokenKind::Comma) {
+                self.advance();
+            }
+            self.skip_newlines();
+        }
+
+        let end = self.current_span();
+        self.expect(TokenKind::RBrace)?;
+
+        Ok(ImplDecl {
+            type_params,
+            constraints,
+            trait_name,
+            trait_args,
+            target_type,
+            methods,
+            span: start.merge(end),
+        })
+    }
+
+    /// Parse trait constraints in a where clause: where T: Show, U: Eq + Ord
+    fn parse_trait_constraints(&mut self) -> Result<Vec<TraitConstraint>, ParseError> {
+        let mut constraints = Vec::new();
+
+        while self.check(TokenKind::Where) {
+            self.advance();
+            let start = self.current_span();
+            let type_param = self.parse_ident()?;
+            self.expect(TokenKind::Colon)?;
+            let bounds = self.parse_trait_bounds()?;
+            let span = start.merge(self.previous_span());
+
+            constraints.push(TraitConstraint {
+                type_param,
+                bounds,
+                span,
+            });
+
+            if self.check(TokenKind::Comma) {
+                self.advance();
+            }
+        }
+
+        Ok(constraints)
+    }
+
+    /// Parse an impl method
+    fn parse_impl_method(&mut self) -> Result<ImplMethod, ParseError> {
+        let start = self.current_span();
+        self.expect(TokenKind::Fn)?;
+
+        let name = self.parse_ident()?;
+
+        self.expect(TokenKind::LParen)?;
+        let params = self.parse_params()?;
+        self.expect(TokenKind::RParen)?;
+
+        // Optional return type (infer if not provided)
+        let return_type = if self.check(TokenKind::Colon) {
+            self.advance();
+            Some(self.parse_type()?)
+        } else {
+            None
+        };
+
+        self.expect(TokenKind::Eq)?;
+        self.skip_newlines();
+        let body = self.parse_expr()?;
+
+        let span = start.merge(body.span());
+        Ok(ImplMethod {
+            name,
+            params,
+            return_type,
+            body,
+            span,
+        })
+    }
+
     /// Parse type parameters <A, B, C>
     fn parse_type_params(&mut self) -> Result<Vec<Ident>, ParseError> {
         self.expect(TokenKind::Lt)?;
diff --git a/src/typechecker.rs b/src/typechecker.rs
index 9c11769..0bd9023 100644
--- a/src/typechecker.rs
+++ b/src/typechecker.rs
@@ -3,16 +3,16 @@
 #![allow(dead_code, unused_variables)]
 
 use crate::ast::{
-    self, BinaryOp, Declaration, EffectDecl, Expr, FunctionDecl, HandlerDecl, Ident, ImportDecl,
-    LetDecl, Literal, LiteralKind, MatchArm, Parameter, Pattern, Program, Span, Statement,
-    TypeDecl, TypeExpr, UnaryOp, VariantFields,
+    self, BinaryOp, Declaration, EffectDecl, Expr, FunctionDecl, HandlerDecl, Ident, ImplDecl,
+    ImportDecl, LetDecl, Literal, LiteralKind, MatchArm, Parameter, Pattern, Program, Span,
+    Statement, TraitDecl, TypeDecl, TypeExpr, UnaryOp, VariantFields,
 };
 use crate::diagnostics::{Diagnostic, Severity};
 use crate::exhaustiveness::{check_exhaustiveness, missing_patterns_hint};
 use crate::modules::ModuleLoader;
 use crate::types::{
-    self, unify, EffectDef, EffectOpDef, EffectSet, HandlerDef, PropertySet, Type, TypeEnv,
-    TypeScheme, VariantDef, VariantFieldsDef,
+    self, unify, EffectDef, EffectOpDef, EffectSet, HandlerDef, PropertySet, TraitBoundDef,
+    TraitDef, TraitImpl, TraitMethodDef, Type, TypeEnv, TypeScheme, VariantDef, VariantFieldsDef,
 };
 
 /// Type checking error
@@ -256,6 +256,15 @@ impl TypeChecker {
                 };
                 self.env.bind(&let_decl.name.name, TypeScheme::mono(typ));
             }
+            Declaration::Trait(trait_decl) => {
+                let trait_def = self.trait_def(trait_decl);
+                self.env.traits.insert(trait_decl.name.name.clone(), trait_def);
+            }
+            Declaration::Impl(impl_decl) => {
+                // Will be checked in second pass
+                let trait_impl = self.collect_impl(impl_decl);
+                self.env.trait_impls.push(trait_impl);
+            }
         }
     }
 
@@ -271,7 +280,10 @@ impl TypeChecker {
             Declaration::Handler(handler) => {
                 self.check_handler(handler);
             }
-            // Effects and types don't need checking beyond collection
+            Declaration::Impl(impl_decl) => {
+                self.check_impl(impl_decl);
+            }
+            // Effects, types, and traits don't need checking beyond collection
             _ => {}
         }
     }
@@ -354,6 +366,29 @@ impl TypeChecker {
                     // For now, we just type-check the predicate expression
                     // (would need 'result' in scope, which we don't have yet)
                 }
+                ast::WhereClause::TraitConstraint(constraint) => {
+                    // Validate that the type parameter exists
+                    if !func.type_params.iter().any(|p| p.name == constraint.type_param.name)
+                        && !func.params.iter().any(|p| p.name.name == constraint.type_param.name)
+                    {
+                        self.errors.push(TypeError {
+                            message: format!(
+                                "Unknown type parameter '{}' in where clause",
+                                constraint.type_param.name
+                            ),
+                            span: constraint.span,
+                        });
+                    }
+                    // Validate that each trait in the bounds exists
+                    for bound in &constraint.bounds {
+                        if !self.env.traits.contains_key(&bound.trait_name.name) {
+                            self.errors.push(TypeError {
+                                message: format!("Unknown trait: {}", bound.trait_name.name),
+                                span: bound.span,
+                            });
+                        }
+                    }
+                }
             }
         }
     }
@@ -1304,6 +1339,165 @@ impl TypeChecker {
         }
     }
 
+    fn trait_def(&self, trait_decl: &TraitDecl) -> TraitDef {
+        let methods = trait_decl
+            .methods
+            .iter()
+            .map(|m| TraitMethodDef {
+                name: m.name.name.clone(),
+                type_params: m.type_params.iter().map(|p| p.name.clone()).collect(),
+                params: m
+                    .params
+                    .iter()
+                    .map(|p| (p.name.name.clone(), self.resolve_type(&p.typ)))
+                    .collect(),
+                return_type: self.resolve_type(&m.return_type),
+                has_default: m.default_impl.is_some(),
+            })
+            .collect();
+
+        let super_traits = trait_decl
+            .super_traits
+            .iter()
+            .map(|b| TraitBoundDef {
+                trait_name: b.trait_name.name.clone(),
+                type_args: b.type_args.iter().map(|t| self.resolve_type(t)).collect(),
+            })
+            .collect();
+
+        TraitDef {
+            name: trait_decl.name.name.clone(),
+            type_params: trait_decl.type_params.iter().map(|p| p.name.clone()).collect(),
+            super_traits,
+            methods,
+        }
+    }
+
+    fn collect_impl(&self, impl_decl: &ImplDecl) -> TraitImpl {
+        use std::collections::HashMap;
+
+        let methods: HashMap<String, Type> = impl_decl
+            .methods
+            .iter()
+            .map(|m| {
+                let return_type = m
+                    .return_type
+                    .as_ref()
+                    .map(|t| self.resolve_type(t))
+                    .unwrap_or_else(Type::var);
+                let param_types: Vec<Type> = m
+                    .params
+                    .iter()
+                    .map(|p| self.resolve_type(&p.typ))
+                    .collect();
+                let func_type = Type::function(param_types, return_type);
+                (m.name.name.clone(), func_type)
+            })
+            .collect();
+
+        let constraints = impl_decl
+            .constraints
+            .iter()
+            .map(|c| {
+                let bounds = c
+                    .bounds
+                    .iter()
+                    .map(|b| TraitBoundDef {
+                        trait_name: b.trait_name.name.clone(),
+                        type_args: b.type_args.iter().map(|t| self.resolve_type(t)).collect(),
+                    })
+                    .collect();
+                (c.type_param.name.clone(), bounds)
+            })
+            .collect();
+
+        TraitImpl {
+            trait_name: impl_decl.trait_name.name.clone(),
+            trait_args: impl_decl
+                .trait_args
+                .iter()
+                .map(|t| self.resolve_type(t))
+                .collect(),
+            target_type: self.resolve_type(&impl_decl.target_type),
+            type_params: impl_decl.type_params.iter().map(|p| p.name.clone()).collect(),
+            constraints,
+            methods,
+        }
+    }
+
+    fn check_impl(&mut self, impl_decl: &ImplDecl) {
+        // Verify the trait exists
+        let trait_name = &impl_decl.trait_name.name;
+        let trait_def = match self.env.traits.get(trait_name) {
+            Some(def) => def.clone(),
+            None => {
+                self.errors.push(TypeError {
+                    message: format!("Unknown trait: {}", trait_name),
+                    span: impl_decl.span,
+                });
+                return;
+            }
+        };
+
+        // Verify all required methods are implemented
+        for method_def in &trait_def.methods {
+            if !method_def.has_default {
+                let implemented = impl_decl.methods.iter().any(|m| m.name.name == method_def.name);
+                if !implemented {
+                    self.errors.push(TypeError {
+                        message: format!(
+                            "Missing implementation for required method '{}' of trait '{}'",
+                            method_def.name, trait_name
+                        ),
+                        span: impl_decl.span,
+                    });
+                }
+            }
+        }
+
+        // Type check each implemented method
+        for impl_method in &impl_decl.methods {
+            // Find the method signature in the trait
+            let method_def = trait_def.methods.iter().find(|m| m.name == impl_method.name.name);
+            if method_def.is_none() {
+                self.errors.push(TypeError {
+                    message: format!(
+                        "Method '{}' is not defined in trait '{}'",
+                        impl_method.name.name, trait_name
+                    ),
+                    span: impl_method.span,
+                });
+                continue;
+            }
+
+            // Set up local environment with parameters
+            let mut local_env = self.env.clone();
+            for param in &impl_method.params {
+                let param_type = self.resolve_type(&param.typ);
+                local_env.bind(&param.name.name, TypeScheme::mono(param_type));
+            }
+
+            // Type check the body
+            let old_env = std::mem::replace(&mut self.env, local_env);
+            let body_type = self.infer_expr(&impl_method.body);
+            self.env = old_env;
+
+            // Check return type matches if specified
+            if let Some(ref return_type_expr) = impl_method.return_type {
+                let return_type = self.resolve_type(return_type_expr);
+                if let Err(e) = unify(&body_type, &return_type) {
+                    self.errors.push(TypeError {
+                        message: format!(
+                            "Method '{}' body has type {}, but declared return type is {}: {}",
+                            impl_method.name.name, body_type, return_type, e
+                        ),
+                        span: impl_method.span,
+                    });
+                }
+            }
+        }
+    }
+
     fn resolve_type(&self, type_expr: &TypeExpr) -> Type {
         match type_expr {
             TypeExpr::Named(ident) => match ident.name.as_str() {
diff --git a/src/types.rs b/src/types.rs
index 2f13ec9..1ece77c 100644
--- a/src/types.rs
+++ b/src/types.rs
@@ -582,6 +582,10 @@ pub struct TypeEnv {
     pub effects: HashMap<String, EffectDef>,
     /// Handler types
     pub handlers: HashMap<String, HandlerDef>,
+    /// Trait definitions
+    pub traits: HashMap<String, TraitDef>,
+    /// Trait implementations: (trait_name, type) -> impl
+    pub trait_impls: Vec<TraitImpl>,
 }
 
 /// Type definition
@@ -615,6 +619,66 @@ pub struct HandlerDef {
     pub params: Vec<(String, Type)>,
 }
 
+/// Trait definition
+#[derive(Debug, Clone)]
+pub struct TraitDef {
+    pub name: String,
+    /// Type parameters for the trait (e.g., Functor<F>)
+    pub type_params: Vec<String>,
+    /// Super traits that must be implemented
+    pub super_traits: Vec<TraitBoundDef>,
+    /// Method signatures
+    pub methods: Vec<TraitMethodDef>,
+}
+
+/// A trait bound in type definitions
+#[derive(Debug, Clone)]
+pub struct TraitBoundDef {
+    pub trait_name: String,
+    pub type_args: Vec<Type>,
+}
+
+/// A method signature in a trait
+#[derive(Debug, Clone)]
+pub struct TraitMethodDef {
+    pub name: String,
+    pub type_params: Vec<String>,
+    pub params: Vec<(String, Type)>,
+    pub return_type: Type,
+    /// Whether this method has a default implementation
+    pub has_default: bool,
+}
+
+/// A trait implementation
+#[derive(Debug, Clone)]
+pub struct TraitImpl {
+    pub trait_name: String,
+    pub trait_args: Vec<Type>,
+    /// The type this impl is for
+    pub target_type: Type,
+    /// Type parameters on the impl (e.g., impl<T> Show for List<T>)
+    pub type_params: Vec<String>,
+    /// Constraints on type parameters (e.g., where T: Show)
+    pub constraints: Vec<(String, Vec<TraitBoundDef>)>,
+    /// Method implementations
+    pub methods: HashMap<String, Type>,
+}
+
+/// A trait constraint on a type variable
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct TraitConstraintDef {
+    pub type_var: String,
+    pub bounds: Vec<TraitBoundDef>,
+}
+
+impl PartialEq for TraitBoundDef {
+    fn eq(&self, other: &Self) -> bool {
+        self.trait_name == other.trait_name && self.type_args == other.type_args
+    }
+}
+
+impl Eq for TraitBoundDef {}
+
 impl TypeEnv {
     pub fn new() -> Self {
         Self::default()