diff --git a/src/lower/datatype.rs b/src/lower/datatype.rs
index f7f24db..6987ca5 100644
--- a/src/lower/datatype.rs
+++ b/src/lower/datatype.rs
@@ -24,6 +24,10 @@ pub fn lower_type(
     match &resolved_type.kind {
         resolved::TypeKind::Unresolved => panic!("got unresolved type during lower_type!"),
         resolved::TypeKind::Polymorph(_, _) => todo!("cannot directly lower polymorph"),
+        resolved::TypeKind::Trait(name, _) => Err(LowerErrorKind::CannotUseTraitDirectly {
+            name: name.to_string(),
+        }
+        .at(resolved_type.source)),
         resolved::TypeKind::Boolean => Ok(ir::Type::Boolean),
         resolved::TypeKind::Integer(bits, sign) => Ok(match (bits, sign) {
             (Bits::Bits8, Sign::Signed) => ir::Type::S8,
diff --git a/src/lower/error.rs b/src/lower/error.rs
index 2b0e4e4..76e9570 100644
--- a/src/lower/error.rs
+++ b/src/lower/error.rs
@@ -41,6 +41,9 @@ pub enum LowerErrorKind {
     },
     PolymorphError(PolymorphErrorKind),
     IncorrectNumberOfTypeArguments,
+    CannotUseTraitDirectly {
+        name: String,
+    },
 }
 
 impl From<PolymorphError> for LowerError {
@@ -106,6 +109,9 @@ impl Display for LowerErrorKind {
                 write!(f, "Incorrect number of type arguments")
             }
             LowerErrorKind::PolymorphError(e) => e.fmt(f),
+            LowerErrorKind::CannotUseTraitDirectly { name } => {
+                write!(f, "Cannot use trait '{}' directly", name)
+            }
         }
     }
 }
diff --git a/src/resolve/error.rs b/src/resolve/error.rs
index 2639bd6..1e6118a 100644
--- a/src/resolve/error.rs
+++ b/src/resolve/error.rs
@@ -206,6 +206,9 @@ pub enum ResolveErrorKind {
     ConstraintsNotSatisfiedForType {
         name: String,
     },
+    TypeIsNotATrait {
+        name: String,
+    },
     Other {
         message: String,
     },
@@ -537,6 +540,11 @@ impl Display for ResolveErrorKind {
             ResolveErrorKind::ConstraintsNotSatisfiedForType { name } => {
                 write!(f, "Constraints not satisfied for type '{}'", name)?;
             }
+            ResolveErrorKind::TypeIsNotATrait {
+                name
+            } => {
+                write!(f, "Type '{}' is not a trait", name)?;
+            }
             ResolveErrorKind::Other { message } => {
                 write!(f, "{}", message)?;
             }
diff --git a/src/resolve/function_head.rs b/src/resolve/function_head.rs
index 3387c1a..af5b131 100644
--- a/src/resolve/function_head.rs
+++ b/src/resolve/function_head.rs
@@ -157,6 +157,7 @@ pub fn collect_constraints(map: &mut HashMap<String, HashSet<Constraint>>, ty: &
                 set.insert(constraint.clone());
             }
         }
+        resolved::TypeKind::Trait(_, _) => (),
     }
 }
 
diff --git a/src/resolve/job.rs b/src/resolve/job.rs
index 0e08eb9..b81b5b1 100644
--- a/src/resolve/job.rs
+++ b/src/resolve/job.rs
@@ -1,5 +1,5 @@
 use crate::{
-    resolved::{self, EnumRef, StructureRef, TypeAliasRef},
+    resolved::{self, EnumRef, StructureRef, TraitRef, TypeAliasRef},
     workspace::fs::FsNodeId,
 };
 
@@ -12,6 +12,7 @@ pub enum FuncJob {
 pub struct TypeJob {
     pub physical_file_id: FsNodeId,
     pub type_aliases: Vec<TypeAliasRef>,
+    pub traits: Vec<TraitRef>,
     pub structures: Vec<StructureRef>,
     pub enums: Vec<EnumRef>,
 }
diff --git a/src/resolve/polymorph.rs b/src/resolve/polymorph.rs
index b6cff6c..faa3b43 100644
--- a/src/resolve/polymorph.rs
+++ b/src/resolve/polymorph.rs
@@ -118,6 +118,7 @@ impl PolyRecipe {
 
                 poly_type.resolved_type.clone()
             }
+            resolved::TypeKind::Trait(_, _) => ty.clone(),
         })
     }
 }
@@ -189,7 +190,8 @@ impl PolyCatalog {
             | resolved::TypeKind::Floating(_)
             | resolved::TypeKind::Void
             | resolved::TypeKind::Enum(_, _)
-            | resolved::TypeKind::TypeAlias(_, _) => {
+            | resolved::TypeKind::TypeAlias(_, _)
+            | resolved::TypeKind::Trait(_, _) => {
                 if *pattern_type == *concrete_type {
                     Ok(())
                 } else {
diff --git a/src/resolve/type_ctx/mod.rs b/src/resolve/type_ctx/mod.rs
index fc0dcfa..d9734db 100644
--- a/src/resolve/type_ctx/mod.rs
+++ b/src/resolve/type_ctx/mod.rs
@@ -55,21 +55,49 @@ impl<'a, 'b, 'c> From<&'c ResolveExprCtx<'a, 'b>> for ResolveTypeCtx<'c> {
     }
 }
 
-pub fn resolve_constraints(constraints: &[ast::Type]) -> Result<Vec<Constraint>, ResolveError> {
+pub fn resolve_constraints(
+    type_ctx: &ResolveTypeCtx,
+    constraints: &[ast::Type],
+) -> Result<Vec<Constraint>, ResolveError> {
     let mut resolved_constraints = vec![];
 
     for constraint in constraints {
-        if let ast::TypeKind::Named(name, arguments) = &constraint.kind {
-            resolved_constraints.push(match name.as_plain_str() {
-                Some("PrimitiveAdd") if arguments.is_empty() => Constraint::PrimitiveAdd,
-                _ => {
-                    return Err(
-                        ResolveErrorKind::UndeclaredTrait(name.to_string()).at(constraint.source)
-                    )
-                }
-            });
-        }
+        resolved_constraints.push(resolve_constraint(type_ctx, constraint)?);
     }
 
     Ok(resolved_constraints)
 }
+
+pub fn resolve_constraint(
+    type_ctx: &ResolveTypeCtx,
+    constraint: &ast::Type,
+) -> Result<Constraint, ResolveError> {
+    if let ast::TypeKind::Named(name, arguments) = &constraint.kind {
+        match name.as_plain_str() {
+            Some("PrimitiveAdd") if arguments.is_empty() => return Ok(Constraint::PrimitiveAdd),
+            _ => {
+                let resolved_type = type_ctx.resolve(constraint).map_err(|err| {
+                    if let ResolveErrorKind::UndeclaredType { name } = err.kind {
+                        ResolveErrorKind::UndeclaredTrait(name).at(err.source)
+                    } else {
+                        err
+                    }
+                })?;
+
+                let resolved::TypeKind::Trait(_, trait_ref) = &resolved_type.kind else {
+                    return Err(ResolveErrorKind::TypeIsNotATrait {
+                        name: resolved_type.to_string(),
+                    }
+                    .at(resolved_type.source));
+                };
+
+                return Ok(Constraint::Trait(*trait_ref));
+            }
+        }
+    }
+
+    return Err(ResolveErrorKind::TypeIsNotATrait {
+        name: constraint.to_string(),
+    }
+    .at(constraint.source));
+}
diff --git a/src/resolve/type_ctx/resolve_type.rs b/src/resolve/type_ctx/resolve_type.rs
index 634fa5f..36fa47f 100644
--- a/src/resolve/type_ctx/resolve_type.rs
+++ b/src/resolve/type_ctx/resolve_type.rs
@@ -110,7 +110,7 @@ impl<'a> ResolveTypeCtx<'a> {
             }
             ast::TypeKind::Polymorph(polymorph, constraints) => Ok(resolved::TypeKind::Polymorph(
                 polymorph.clone(),
-                resolve_constraints(constraints)?,
+                resolve_constraints(self, constraints)?,
             )),
         }
         .map(|kind| kind.at(ast_type.source))
diff --git a/src/resolve/type_definition/prepare.rs b/src/resolve/type_definition/prepare.rs
index 4af1915..0e634e6 100644
--- a/src/resolve/type_definition/prepare.rs
+++ b/src/resolve/type_definition/prepare.rs
@@ -5,9 +5,12 @@ use crate::{
         ctx::ResolveCtx,
         error::{ResolveError, ResolveErrorKind},
         job::TypeJob,
-        type_ctx::resolve_constraints,
+        type_ctx::{resolve_constraints, ResolveTypeCtx},
+    },
+    resolved::{
+        self, CurrentConstraints, EnumRef, HumanName, StructureRef, TraitRef, TypeAliasRef,
+        TypeDecl, TypeParameters,
     },
-    resolved::{self, EnumRef, HumanName, StructureRef, TypeAliasRef, TypeDecl, TypeParameters},
     workspace::fs::FsNodeId,
 };
 use indexmap::IndexMap;
@@ -28,15 +31,26 @@ pub fn prepare_type_jobs(
         let mut job = TypeJob {
             physical_file_id: *physical_file_id,
             type_aliases: Vec::with_capacity(file.type_aliases.len()),
+            traits: Vec::with_capacity(file.traits.len()),
             structures: Vec::with_capacity(file.structures.len()),
             enums: Vec::with_capacity(file.enums.len()),
         };
 
+        for user_trait in file.traits.iter() {
+            job.traits.push(prepare_trait(
+                ctx,
+                resolved_ast,
+                module_fs_node_id,
+                user_trait,
+            ));
+        }
+
         for structure in file.structures.iter() {
             job.structures.push(prepare_structure(
                 ctx,
                 resolved_ast,
                 module_fs_node_id,
+                *physical_file_id,
                 structure,
             )?);
         }
@@ -69,6 +83,7 @@ fn prepare_structure(
     ctx: &mut ResolveCtx,
     resolved_ast: &mut resolved::Ast,
     module_fs_node_id: FsNodeId,
+    physical_fs_node_id: FsNodeId,
     structure: &ast::Structure,
 ) -> Result<StructureRef, ResolveError> {
     let source = structure.source;
@@ -76,7 +91,17 @@ fn prepare_structure(
     let mut parameters = TypeParameters::default();
 
     for (name, parameter) in structure.parameters.iter() {
-        let constraints = resolve_constraints(&parameter.constraints)?;
+        let zero_current_constraints = CurrentConstraints::default();
+        let constraints = resolve_constraints(
+            &ResolveTypeCtx::new(
+                resolved_ast,
+                module_fs_node_id,
+                physical_fs_node_id,
+                &ctx.types_in_modules,
+                &zero_current_constraints,
+            ),
+            &parameter.constraints,
+        )?;
 
         if parameters
             .parameters
@@ -155,6 +180,36 @@ fn prepare_enum(
     enum_ref
 }
 
+fn prepare_trait(
+    ctx: &mut ResolveCtx,
+    resolved_ast: &mut resolved::Ast,
+    module_fs_node_id: FsNodeId,
+    definition: &ast::Trait,
+) -> TraitRef {
+    let trait_ref = resolved_ast.traits.insert(resolved::Trait {
+        methods: vec![],
+        source: definition.source,
+    });
+
+    let kind = resolved::TypeKind::Trait(HumanName(definition.name.to_string()), trait_ref);
+    let source = definition.source;
+    let privacy = definition.privacy;
+
+    ctx.types_in_modules
+        .entry(module_fs_node_id)
+        .or_default()
+        .insert(
+            definition.name.to_string(),
+            TypeDecl {
+                kind,
+                source,
+                privacy,
+            },
+        );
+
+    trait_ref
+}
+
 fn prepare_type_alias(
     ctx: &mut ResolveCtx,
     resolved_ast: &mut resolved::Ast,
diff --git a/src/resolve/type_definition/resolve.rs b/src/resolve/type_definition/resolve.rs
index 1f49825..ba4733f 100644
--- a/src/resolve/type_definition/resolve.rs
+++ b/src/resolve/type_definition/resolve.rs
@@ -30,6 +30,10 @@ pub fn resolve_type_jobs(
             .get_owning_module(job.physical_file_id)
             .unwrap_or(job.physical_file_id);
 
+        for (_trait_ref, _user_trait) in job.traits.iter().zip(file.traits.iter()) {
+            resolve_trait()?;
+        }
+
         for (structure_ref, structure) in job.structures.iter().zip(file.structures.iter()) {
             resolve_structure(
                 ctx,
@@ -76,12 +80,22 @@ fn resolve_structure(
     structure_ref: StructureRef,
 ) -> Result<(), ResolveError> {
     for (field_name, field) in structure.fields.iter() {
-        let mut constraints = HashMap::new();
+        let pre_constraints = CurrentConstraints::default();
+        let pre_type_ctx = ResolveTypeCtx::new(
+            &resolved_ast,
+            module_file_id,
+            physical_file_id,
+            &ctx.types_in_modules,
+            &pre_constraints,
+        );
 
+        let mut constraints = HashMap::new();
         for (name, parameter) in structure.parameters.iter() {
             constraints.insert(
                 name.into(),
-                HashSet::from_iter(resolve_constraints(&parameter.constraints)?.drain(..)),
+                HashSet::from_iter(
+                    resolve_constraints(&pre_type_ctx, &parameter.constraints)?.drain(..),
+                ),
             );
         }
 
@@ -165,3 +179,8 @@ fn resolve_type_alias(
     *resolved_ast.type_aliases.get_mut(type_alias_ref).unwrap() = resolved_type;
     Ok(())
 }
+
+fn resolve_trait() -> Result<(), ResolveError> {
+    eprintln!("warning: trait methods are not resolved yet");
+    Ok(())
+}
diff --git a/src/resolved/datatype/kind/constraint.rs b/src/resolved/datatype/kind/constraint.rs
index ebd46a6..5398c87 100644
--- a/src/resolved/datatype/kind/constraint.rs
+++ b/src/resolved/datatype/kind/constraint.rs
@@ -1,14 +1,17 @@
+use crate::resolved::TraitRef;
 use std::fmt::Display;
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub enum Constraint {
     PrimitiveAdd,
+    Trait(TraitRef),
 }
 
 impl Display for Constraint {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
             Constraint::PrimitiveAdd => write!(f, "PrimitiveAdd"),
+            Constraint::Trait(_) => write!(f, "<user-defined trait>"),
         }
     }
 }
diff --git a/src/resolved/datatype/kind/mod.rs b/src/resolved/datatype/kind/mod.rs
index 9b80215..ed109ab 100644
--- a/src/resolved/datatype/kind/mod.rs
+++ b/src/resolved/datatype/kind/mod.rs
@@ -6,7 +6,7 @@ mod function_pointer;
 use super::Type;
 use crate::{
     ast::{fmt_c_integer, CInteger, FloatSize, IntegerBits, IntegerSign},
-    resolved::{human_name::HumanName, Ast, EnumRef, StructureRef, TypeAliasRef},
+    resolved::{human_name::HumanName, Ast, EnumRef, StructureRef, TraitRef, TypeAliasRef},
     source_files::Source,
     target::Target,
 };
@@ -39,6 +39,7 @@ pub enum TypeKind {
     Structure(HumanName, StructureRef, Vec<Type>),
     TypeAlias(HumanName, TypeAliasRef),
     Polymorph(String, Vec<Constraint>),
+    Trait(HumanName, TraitRef),
 }
 
 impl TypeKind {
@@ -70,6 +71,7 @@ impl TypeKind {
                 .any(|parameter| parameter.kind.contains_polymorph()),
             TypeKind::TypeAlias(_, _) => false,
             TypeKind::Polymorph(_, _) => true,
+            TypeKind::Trait(_, _) => false,
         }
     }
 
@@ -98,7 +100,8 @@ impl TypeKind {
             | TypeKind::FunctionPointer(..)
             | TypeKind::Enum(_, _)
             | TypeKind::AnonymousEnum()
-            | TypeKind::Polymorph(_, _) => None,
+            | TypeKind::Polymorph(_, _)
+            | TypeKind::Trait(_, _) => None,
         }
     }
 
@@ -190,6 +193,9 @@ impl Display for TypeKind {
                     write!(f, "{:?}", constaint)?;
                 }
             }
+            TypeKind::Trait(name, _) => {
+                write!(f, "{}", name)?;
+            }
         }
 
         Ok(())
diff --git a/src/resolved/datatype/mod.rs b/src/resolved/datatype/mod.rs
index a8ce080..57e21c1 100644
--- a/src/resolved/datatype/mod.rs
+++ b/src/resolved/datatype/mod.rs
@@ -55,6 +55,7 @@ impl Type {
             TypeKind::Polymorph(_, constraints) => {
                 constraints.drain(..);
             }
+            TypeKind::Trait(_, _) => (),
         }
     }
 }
diff --git a/src/resolved/function.rs b/src/resolved/function.rs
index 6f75b03..1369a8e 100644
--- a/src/resolved/function.rs
+++ b/src/resolved/function.rs
@@ -9,7 +9,7 @@ pub struct CurrentConstraints {
 impl CurrentConstraints {
     pub fn satisfies(&self, ty: &Type, constraint: &Constraint) -> bool {
         match constraint {
-            Constraint::PrimitiveAdd => match &ty.kind {
+            Constraint::PrimitiveAdd | Constraint::Trait(..) => match &ty.kind {
                 TypeKind::Integer(..) | TypeKind::CInteger(..) | TypeKind::Floating(..) => true,
                 TypeKind::Polymorph(name, constraints) => {
                     constraints.contains(constraint)
diff --git a/src/resolved/mod.rs b/src/resolved/mod.rs
index 6f406da..f108b0c 100644
--- a/src/resolved/mod.rs
+++ b/src/resolved/mod.rs
@@ -10,6 +10,7 @@ mod human_name;
 mod overload;
 mod stmt;
 mod structure;
+mod trait_constraint;
 mod type_decl;
 mod variable_storage;
 
@@ -30,6 +31,7 @@ use slotmap::{new_key_type, SlotMap};
 use std::collections::HashMap;
 pub use stmt::*;
 pub use structure::*;
+pub use trait_constraint::Trait;
 pub use type_decl::*;
 pub use variable_storage::*;
 
@@ -39,6 +41,7 @@ new_key_type! {
     pub struct StructureRef;
     pub struct EnumRef;
     pub struct TypeAliasRef;
+    pub struct TraitRef;
 }
 
 #[derive(Clone, Debug)]
@@ -50,6 +53,7 @@ pub struct Ast<'a> {
     pub globals: SlotMap<GlobalVarRef, GlobalVar>,
     pub enums: SlotMap<EnumRef, Enum>,
     pub type_aliases: SlotMap<TypeAliasRef, Type>,
+    pub traits: SlotMap<TraitRef, Trait>,
     pub workspace: &'a AstWorkspace<'a>,
 }
 
@@ -65,6 +69,7 @@ impl<'a> Ast<'a> {
             globals: SlotMap::with_key(),
             enums: SlotMap::with_key(),
             type_aliases: SlotMap::with_key(),
+            traits: SlotMap::with_key(),
             workspace,
         }
     }
diff --git a/src/resolved/trait_constraint.rs b/src/resolved/trait_constraint.rs
new file mode 100644
index 0000000..c587645
--- /dev/null
+++ b/src/resolved/trait_constraint.rs
@@ -0,0 +1,16 @@
+use super::{Parameters, Type};
+use crate::source_files::Source;
+
+#[derive(Clone, Debug)]
+pub struct Trait {
+    pub source: Source,
+    pub methods: Vec<TraitMethod>,
+}
+
+#[derive(Clone, Debug)]
+pub struct TraitMethod {
+    pub name: String,
+    pub parameters: Parameters,
+    pub return_type: Type,
+    pub source: Source,
+}