diff --git a/eth2/state_processing/src/lib.rs b/eth2/state_processing/src/lib.rs
index 44c0e28a0..2b30844cb 100644
--- a/eth2/state_processing/src/lib.rs
+++ b/eth2/state_processing/src/lib.rs
@@ -3,12 +3,11 @@ mod macros;
 
 pub mod per_block_processing;
 pub mod per_epoch_processing;
-// mod slot_processable;
+pub mod per_slot_processing;
 
 pub use per_block_processing::{
     errors::{BlockInvalid, BlockProcessingError},
     per_block_processing, per_block_processing_without_verifying_block_signature,
 };
 pub use per_epoch_processing::{errors::EpochProcessingError, per_epoch_processing};
-// pub use epoch_processable::{EpochProcessable, Error as EpochProcessingError};
-// pub use slot_processable::{Error as SlotProcessingError, SlotProcessable};
+pub use per_slot_processing::{per_slot_processing, Error as SlotProcessingError};
diff --git a/eth2/state_processing/src/per_slot_processing.rs b/eth2/state_processing/src/per_slot_processing.rs
new file mode 100644
index 000000000..0bb405c98
--- /dev/null
+++ b/eth2/state_processing/src/per_slot_processing.rs
@@ -0,0 +1,58 @@
+use crate::*;
+use types::{BeaconState, BeaconStateError, ChainSpec, Hash256};
+
+#[derive(Debug, PartialEq)]
+pub enum Error {
+    BeaconStateError(BeaconStateError),
+    EpochProcessingError(EpochProcessingError),
+}
+
+/// Advances a state forward by one slot, performing per-epoch processing if required.
+///
+/// Spec v0.4.0
+pub fn per_slot_processing(
+    state: &mut BeaconState,
+    previous_block_root: Hash256,
+    spec: &ChainSpec,
+) -> Result<(), Error> {
+    if (state.slot + 1) % spec.slots_per_epoch == 0 {
+        per_epoch_processing(state, spec)?;
+        state.advance_caches();
+    }
+
+    state.slot += 1;
+
+    update_block_roots(state, previous_block_root, spec);
+
+    Ok(())
+}
+
+/// Updates the state's block roots as per-slot processing is performed.
+///
+/// Spec v0.4.0
+pub fn update_block_roots(state: &mut BeaconState, previous_block_root: Hash256, spec: &ChainSpec) {
+    state.latest_block_roots[(state.slot.as_usize() - 1) % spec.latest_block_roots_length] =
+        previous_block_root;
+
+    if state.slot.as_usize() % spec.latest_block_roots_length == 0 {
+        let root = merkle_root(&state.latest_block_roots[..]);
+        state.batched_block_roots.push(root);
+    }
+}
+
+fn merkle_root(_input: &[Hash256]) -> Hash256 {
+    // TODO: implement correctly.
+    Hash256::zero()
+}
+
+impl From<BeaconStateError> for Error {
+    fn from(e: BeaconStateError) -> Error {
+        Error::BeaconStateError(e)
+    }
+}
+
+impl From<EpochProcessingError> for Error {
+    fn from(e: EpochProcessingError) -> Error {
+        Error::EpochProcessingError(e)
+    }
+}
diff --git a/eth2/state_processing/src/slot_processable.rs b/eth2/state_processing/src/slot_processable.rs
deleted file mode 100644
index 8d6506c36..000000000
--- a/eth2/state_processing/src/slot_processable.rs
+++ /dev/null
@@ -1,71 +0,0 @@
-use crate::{EpochProcessable, EpochProcessingError};
-use types::{BeaconState, BeaconStateError, ChainSpec, Hash256};
-
-#[derive(Debug, PartialEq)]
-pub enum Error {
-    BeaconStateError(BeaconStateError),
-    EpochProcessingError(EpochProcessingError),
-}
-
-pub trait SlotProcessable {
-    fn per_slot_processing(
-        &mut self,
-        previous_block_root: Hash256,
-        spec: &ChainSpec,
-    ) -> Result<(), Error>;
-}
-
-impl SlotProcessable for BeaconState
-where
-    BeaconState: EpochProcessable,
-{
-    fn per_slot_processing(
-        &mut self,
-        previous_block_root: Hash256,
-        spec: &ChainSpec,
-    ) -> Result<(), Error> {
-        if (self.slot + 1) % spec.slots_per_epoch == 0 {
-            self.per_epoch_processing(spec)?;
-            self.advance_caches();
-        }
-
-        self.slot += 1;
-
-        self.latest_randao_mixes[self.slot.as_usize() % spec.latest_randao_mixes_length] =
-            self.latest_randao_mixes[(self.slot.as_usize() - 1) % spec.latest_randao_mixes_length];
-
-        // Block roots.
-        self.latest_block_roots[(self.slot.as_usize() - 1) % spec.latest_block_roots_length] =
-            previous_block_root;
-
-        if self.slot.as_usize() % spec.latest_block_roots_length == 0 {
-            let root = merkle_root(&self.latest_block_roots[..]);
-            self.batched_block_roots.push(root);
-        }
-        Ok(())
-    }
-}
-
-fn merkle_root(_input: &[Hash256]) -> Hash256 {
-    Hash256::zero()
-}
-
-impl From<BeaconStateError> for Error {
-    fn from(e: BeaconStateError) -> Error {
-        Error::BeaconStateError(e)
-    }
-}
-
-impl From<EpochProcessingError> for Error {
-    fn from(e: EpochProcessingError) -> Error {
-        Error::EpochProcessingError(e)
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    #[test]
-    fn it_works() {
-        assert_eq!(2 + 2, 4);
-    }
-}