diff --git a/src/lib.rs b/src/lib.rs
index d288c43..f8d3e55 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -3,3 +3,4 @@
 //! full design.
 
 pub mod core;
+pub mod traits;
diff --git a/src/traits/initializer.rs b/src/traits/initializer.rs
new file mode 100644
index 0000000..2734f5e
--- /dev/null
+++ b/src/traits/initializer.rs
@@ -0,0 +1,12 @@
+//! Trait for sampling initial decisions.
+
+use crate::core::rng::Rng;
+
+/// Generates initial decisions for a population.
+///
+/// Implementations should return exactly `size` decisions; if that is
+/// impossible, panic with a clear message in v1.
+pub trait Initializer<D> {
+    /// Produce `size` initial decisions using the supplied RNG.
+    fn initialize(&mut self, size: usize, rng: &mut Rng) -> Vec<D>;
+}
diff --git a/src/traits/mod.rs b/src/traits/mod.rs
new file mode 100644
index 0000000..faeb203
--- /dev/null
+++ b/src/traits/mod.rs
@@ -0,0 +1,9 @@
+//! The small set of traits that user code and built-in algorithms implement.
+
+pub mod initializer;
+pub mod optimizer;
+pub mod variation;
+
+pub use initializer::*;
+pub use optimizer::*;
+pub use variation::*;
diff --git a/src/traits/optimizer.rs b/src/traits/optimizer.rs
new file mode 100644
index 0000000..2a8ac76
--- /dev/null
+++ b/src/traits/optimizer.rs
@@ -0,0 +1,18 @@
+//! The single trait users implement to add a new optimizer.
+
+use crate::core::problem::Problem;
+use crate::core::result::OptimizationResult;
+
+/// An optimizer that runs to completion in a single call.
+///
+/// Implementations own their main loop, manage their own state, and return an
+/// [`OptimizationResult`]. v1 deliberately does not expose a step-by-step API
+/// or an associated error type — invalid configuration may panic with a clear
+/// message.
+pub trait Optimizer<P>
+where
+    P: Problem,
+{
+    /// Run the optimizer to completion against `problem`.
+    fn run(&mut self, problem: &P) -> OptimizationResult<P::Decision>;
+}
diff --git a/src/traits/variation.rs b/src/traits/variation.rs
new file mode 100644
index 0000000..42530c7
--- /dev/null
+++ b/src/traits/variation.rs
@@ -0,0 +1,13 @@
+//! Trait for producing child decisions from parents.
+
+use crate::core::rng::Rng;
+
+/// Generates child decisions from parent decisions.
+///
+/// The optimizer chooses how many parents to pass. Implementations may return
+/// any number of children; algorithms that require a specific count should
+/// panic with a clear message if they do not get it.
+pub trait Variation<D> {
+    /// Produce children from the given parents.
+    fn vary(&mut self, parents: &[D], rng: &mut Rng) -> Vec<D>;
+}