diff --git a/src/lib.rs b/src/lib.rs
index a4d74e1..8619304 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,40 +1,14 @@
 //! Cyborg is a high-performance, modern, experimental rendering engine written
 //! in Rust.
 
-use std::sync::{Arc, RwLock};
 use strum::IntoEnumIterator;
 
 pub mod phase;
+pub mod pass;
 
+use pass::*;
 use phase::*;
 
-pub struct ViewportData;
-
-pub trait FrameData {}
-
-pub trait RenderPass {
-    type FrameData: FrameData;
-
-    fn create_frame_data(&mut self) -> Self::FrameData;
-
-    fn begin_frame(&mut self, data: &mut Self::FrameData, phases: &mut PhaseList);
-
-    fn render_pre(
-        &self,
-        phase: PrePhase,
-        data: &Self::FrameData,
-        cmds: &mut gpu::RenderBundleEncoder,
-    );
-
-    fn render_viewport(
-        &self,
-        phase: ViewportPhase,
-        data: &Self::FrameData,
-        viewport: &ViewportData,
-        cmds: &mut gpu::RenderBundleEncoder,
-    );
-}
-
 pub struct Renderer {
     pub render_passes: Vec<Box<dyn RenderPassBoxTrait>>,
 }
@@ -72,65 +46,6 @@ impl Renderer {
     }
 }
 
-pub trait RenderPassBoxTrait {
-    fn begin_frame(&mut self, data_index: usize, phases: &mut PhaseList);
-
-    fn render_pre(&self, phase: PrePhase, data_index: usize, cmds: &mut gpu::RenderBundleEncoder);
-
-    fn render_viewport(
-        &self,
-        phase: ViewportPhase,
-        data_index: usize,
-        viewport: &ViewportData,
-        cmds: &mut gpu::RenderBundleEncoder,
-    );
-}
-
-pub struct RenderPassBox<T: RenderPass> {
-    render_pass: Arc<RwLock<T>>,
-    frame_data: Vec<T::FrameData>,
-}
-
-impl<T: RenderPass> RenderPassBox<T> {
-    pub fn new(render_pass: Arc<RwLock<T>>, frame_num: usize) -> Self {
-        let frame_data = {
-            let mut lock = render_pass.write().unwrap();
-            (0..frame_num).map(|_| lock.create_frame_data()).collect()
-        };
-
-        Self {
-            render_pass,
-            frame_data,
-        }
-    }
-}
-
-impl<T: RenderPass> RenderPassBoxTrait for RenderPassBox<T> {
-    fn begin_frame(&mut self, data_index: usize, phases: &mut PhaseList) {
-        let frame_data = &mut self.frame_data[data_index];
-        let mut render_pass = self.render_pass.write().unwrap();
-        render_pass.begin_frame(frame_data, phases)
-    }
-
-    fn render_pre(&self, phase: PrePhase, data_index: usize, cmds: &mut gpu::RenderBundleEncoder) {
-        let frame_data = &self.frame_data[data_index];
-        let render_pass = self.render_pass.read().unwrap();
-        render_pass.render_pre(phase, frame_data, cmds)
-    }
-
-    fn render_viewport(
-        &self,
-        phase: ViewportPhase,
-        data_index: usize,
-        viewport: &ViewportData,
-        cmds: &mut gpu::RenderBundleEncoder,
-    ) {
-        let frame_data = &self.frame_data[data_index];
-        let render_pass = self.render_pass.read().unwrap();
-        render_pass.render_viewport(phase, frame_data, viewport, cmds)
-    }
-}
-
 /// Mock GPU API (like Vulkan, WebGPU, or OpenGL) types for prototyping.
 pub mod gpu {
     /// Thread-safe GPU command recorder that's later executed in a command buffer.
diff --git a/src/pass.rs b/src/pass.rs
new file mode 100644
index 0000000..c18a80e
--- /dev/null
+++ b/src/pass.rs
@@ -0,0 +1,122 @@
+//! Render pass data structures and interfaces.
+
+use crate::gpu;
+use crate::phase::*;
+use std::sync::{Arc, RwLock};
+
+/// Viewport data shared by all passes, once the [ViewportPhase] group is
+/// entered.
+pub struct ViewportData;
+
+/// The render pass interface trait.
+///
+/// Render passes are persistent structures that share GPU resources, user data,
+/// and more across both phases and frames.
+///
+/// Rendering functions record their GPU commands using the [gpu::RenderBundleEncoder]
+/// passed to them.
+pub trait RenderPass {
+    /// A structure that contains a pass's per-frame data.
+    type FrameData;
+
+    /// Sets up a new instance of [Self::FrameData].
+    ///
+    /// Called when a render pass is added to [crate::Renderer].
+    fn create_frame_data(&mut self) -> Self::FrameData;
+
+    /// Initializes this frame's [Self::FrameData], and queries the pass for
+    /// which phases to execute.
+    ///
+    /// This is the only opportunity the render pass has to mutate this frame's
+    /// data this frame, so all setup for future phases should be done here.
+    fn begin_frame(&mut self, data: &mut Self::FrameData, phases: &mut PhaseList);
+
+    /// Renders a phase in the [PrePhase] group.
+    fn render_pre(
+        &self,
+        phase: PrePhase,
+        data: &Self::FrameData,
+        cmds: &mut gpu::RenderBundleEncoder,
+    );
+
+    /// Renders a phase in the [ViewportPhase] group.
+    ///
+    /// During these render phases, passes may have access to this frame's
+    /// [ViewportData].
+    fn render_viewport(
+        &self,
+        phase: ViewportPhase,
+        data: &Self::FrameData,
+        viewport: &ViewportData,
+        cmds: &mut gpu::RenderBundleEncoder,
+    );
+}
+
+/// The interface trait for [RenderPassBox], allowing use of a statically-sized
+/// [Box] for storage.
+///
+/// Functions on this trait map one-to-one with [RenderPass], except that
+/// frame data is passed by index and not by reference.
+pub trait RenderPassBoxTrait {
+    fn begin_frame(&mut self, data_index: usize, phases: &mut PhaseList);
+
+    fn render_pre(&self, phase: PrePhase, data_index: usize, cmds: &mut gpu::RenderBundleEncoder);
+
+    fn render_viewport(
+        &self,
+        phase: ViewportPhase,
+        data_index: usize,
+        viewport: &ViewportData,
+        cmds: &mut gpu::RenderBundleEncoder,
+    );
+}
+
+/// A container for a reference-counted render pass instance and its frame data.
+pub struct RenderPassBox<T: RenderPass> {
+    render_pass: Arc<RwLock<T>>,
+    frame_data: Vec<T::FrameData>,
+}
+
+impl<T: RenderPass> RenderPassBox<T> {
+    /// Creates a new boxed render pass.
+    ///
+    /// Calls to [RenderPassBoxTrait] functions with frame indices greater
+    /// than or equal to `frame_num` are out-of-bounds and will panic.
+    pub fn new(render_pass: Arc<RwLock<T>>, frame_num: usize) -> Self {
+        let frame_data = {
+            let mut lock = render_pass.write().unwrap();
+            (0..frame_num).map(|_| lock.create_frame_data()).collect()
+        };
+
+        Self {
+            render_pass,
+            frame_data,
+        }
+    }
+}
+
+impl<T: RenderPass> RenderPassBoxTrait for RenderPassBox<T> {
+    fn begin_frame(&mut self, data_index: usize, phases: &mut PhaseList) {
+        let frame_data = &mut self.frame_data[data_index];
+        let mut render_pass = self.render_pass.write().unwrap();
+        render_pass.begin_frame(frame_data, phases)
+    }
+
+    fn render_pre(&self, phase: PrePhase, data_index: usize, cmds: &mut gpu::RenderBundleEncoder) {
+        let frame_data = &self.frame_data[data_index];
+        let render_pass = self.render_pass.read().unwrap();
+        render_pass.render_pre(phase, frame_data, cmds)
+    }
+
+    fn render_viewport(
+        &self,
+        phase: ViewportPhase,
+        data_index: usize,
+        viewport: &ViewportData,
+        cmds: &mut gpu::RenderBundleEncoder,
+    ) {
+        let frame_data = &self.frame_data[data_index];
+        let render_pass = self.render_pass.read().unwrap();
+        render_pass.render_viewport(phase, frame_data, viewport, cmds)
+    }
+}
diff --git a/src/phase.rs b/src/phase.rs
index a985194..4a7201f 100644
--- a/src/phase.rs
+++ b/src/phase.rs
@@ -1,4 +1,4 @@
-//! Definitions and containers for rendering phase identifiers.
+//! Definitions and containers for render phase identifiers.
 //!
 //! Rendering phases are organized into different groups, which represent
 //! different kinds of external data becoming available as the frame progresses.