diff --git a/nomt/src/lib.rs b/nomt/src/lib.rs
index b9567d7a..1eb5ad4b 100644
--- a/nomt/src/lib.rs
+++ b/nomt/src/lib.rs
@@ -38,6 +38,7 @@ mod bitbox;
 mod merkle;
 mod metrics;
 mod options;
+mod overlay;
 mod page_cache;
 mod page_diff;
 mod page_region;
diff --git a/nomt/src/overlay.rs b/nomt/src/overlay.rs
new file mode 100644
index 00000000..24aaf34a
--- /dev/null
+++ b/nomt/src/overlay.rs
@@ -0,0 +1,265 @@
+//! A utility for managing in-memory overlays.
+//!
+//! This module exposes two types. The user-facing [`Overlay`] is opaque and frozen. The internal
+//! [`LiveOverlay`] is meant to be used within a session only.
+//!
+//! Overlays contain weak references to all their ancestors. This allows ancestors to be dropped or
+//! committed during the lifetime of the overlay. Importantly, this means that memory is cleaned
+//! up gracefully as overlays are dropped and committed.
+//!
+//! However, creating a new [`LiveOverlay`] requires the user to provide strong references to each
+//! of the ancestors which are still alive. It is still the user's responsibility to ensure
+//! that all live ancestors are provided, or else data will go silently missing.
+//!
+//! Looking up a value in an overlay does not have a varying cost. First there is a look-up in an
+//! index to see which ancestor has the data, and then a query on the ancestor's storage is done.
+//!
+//! Creating a new overlay is an O(n) operation in the amount of changes relative to the parent,
+//! both in terms of new changes and outdated ancestors.
+
+#![allow(dead_code)]
+
+use crate::{beatree::ValueChange, page_cache::Page, page_diff::PageDiff};
+use nomt_core::{page_id::PageId, trie::KeyPath};
+
+use std::collections::HashMap;
+use std::sync::{Arc, Weak};
+
+/// An in-memory overlay of merkle tree and b-tree changes.
+pub struct Overlay {
+    inner: Arc<OverlayInner>,
+}
+
+struct OverlayInner {
+    index: Index,
+    data: Arc<Data>,
+    seqn: u64,
+    // ordered by recency.
+    ancestor_data: Vec<Weak<Data>>,
+}
+
+// Maps changes to sequence number.
+#[derive(Default, Clone)]
+struct Index {
+    pages: imbl::HashMap<PageId, u64>,
+    values: imbl::OrdMap<KeyPath, u64>,
+
+    // sorted ascending by seqn.
+    pages_by_seqn: imbl::Vector<(u64, PageId)>,
+    values_by_seqn: imbl::Vector<(u64, KeyPath)>,
+}
+
+impl Index {
+    // Prune all items with a sequence number less than the minimum.
+    // O(n) in number of pruned items.
+    fn prune_below(&mut self, min: u64) {
+        loop {
+            match self.pages_by_seqn.pop_front() {
+                None => break,
+                Some((seqn, key)) if seqn >= min => {
+                    self.pages_by_seqn.push_front((seqn, key));
+                    break;
+                }
+                Some((seqn, page_id)) => {
+                    if let Some(got_seqn) = self
+                        .pages
+                        .remove(&page_id)
+                        .filter(|&got_seqn| got_seqn != seqn && got_seqn >= min)
+                    {
+                        // page_id has been updated since this point. reinsert.
+                        self.pages.insert(page_id, got_seqn);
+                    }
+                }
+            }
+        }
+
+        loop {
+            match self.values_by_seqn.pop_front() {
+                None => break,
+                Some((seqn, key)) if seqn >= min => {
+                    self.values_by_seqn.push_front((seqn, key));
+                    break;
+                }
+                Some((seqn, key)) => {
+                    if let Some(got_seqn) = self
+                        .values
+                        .remove(&key)
+                        .filter(|&got_seqn| got_seqn != seqn && got_seqn >= min)
+                    {
+                        // key has been updated since this point. reinsert.
+                        self.values.insert(key, got_seqn);
+                    }
+                }
+            }
+        }
+    }
+
+    /// Insert all the value keys in the iterator with the given sequence number.
+    ///
+    /// The sequence number is assumed to be greater than or equal to the maximum in the vector.
+    fn insert_pages(&mut self, seqn: u64, page_ids: impl IntoIterator<Item = PageId>) {
+        for page_id in page_ids {
+            self.pages_by_seqn.push_back((seqn, page_id.clone()));
+            self.pages.insert(page_id, seqn);
+        }
+    }
+
+    /// Insert all the value keys in the iterator with the given sequence number.
+    ///
+    /// The sequence number is assumed to be greater than or equal to the maximum in the vector.
+    fn insert_values(&mut self, seqn: u64, value_keys: impl IntoIterator<Item = KeyPath>) {
+        for key in value_keys {
+            self.values_by_seqn.push_back((seqn, key));
+            self.values.insert(key, seqn);
+        }
+    }
+}
+
+/// Data associated with a single overlay.
+struct Data {
+    pages: HashMap<PageId, (Page, PageDiff)>,
+    values: HashMap<KeyPath, ValueChange>,
+}
+
+/// An error type indicating that the ancestors provided did not match.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct InvalidAncestors;
+
+/// A live overlay which is being used as a child. This can be queried for all value/page changes in
+/// any of the relevant ancestors.
+pub(super) struct LiveOverlay {
+    parent: Option<Arc<OverlayInner>>,
+    ancestor_data: Vec<Arc<Data>>,
+    min_seqn: u64,
+}
+
+impl LiveOverlay {
+    /// Create a new live overlay based on this iterator of ancestors.
+    pub(super) fn new<'a>(
+        live_ancestors: impl IntoIterator<Item = &'a Overlay>,
+    ) -> Result<Self, InvalidAncestors> {
+        let mut live_ancestors = live_ancestors.into_iter();
+        let Some(parent) = live_ancestors.next().map(|p| p.inner.clone()) else {
+            return Ok(LiveOverlay {
+                parent: None,
+                ancestor_data: Vec::new(),
+                min_seqn: 0,
+            });
+        };
+
+        let mut ancestor_data = Vec::new();
+        for (supposed_ancestor, actual_ancestor) in live_ancestors.zip(parent.ancestor_data.iter())
+        {
+            let Some(actual_ancestor) = actual_ancestor.upgrade() else {
+                return Err(InvalidAncestors);
+            };
+
+            if !Arc::ptr_eq(&supposed_ancestor.inner.data, &actual_ancestor) {
+                return Err(InvalidAncestors);
+            }
+
+            ancestor_data.push(actual_ancestor);
+        }
+
+        let min_seqn = parent.seqn - ancestor_data.len() as u64;
+
+        Ok(LiveOverlay {
+            parent: Some(parent),
+            ancestor_data,
+            min_seqn,
+        })
+    }
+
+    /// Get a page by ID.
+    ///
+    /// `None` indicates that the page is not present in the overlay, not that the page doesn't
+    /// exist.
+    pub(super) fn page(&self, page_id: &PageId) -> Option<(Page, PageDiff)> {
+        self.parent
+            .as_ref()
+            .and_then(|parent| parent.index.pages.get(&page_id))
+            .and_then(|seqn| seqn.checked_sub(self.min_seqn))
+            .map(|seqn_diff| {
+                if seqn_diff == 0 {
+                    self.parent
+                        .as_ref()
+                        .unwrap() // UNWRAP: parent existence checked above
+                        .data
+                        .pages
+                        .get(page_id)
+                        .unwrap() // UNWRAP: index indicates that data exists.
+                } else {
+                    self.ancestor_data[seqn_diff as usize - 1]
+                        .pages
+                        .get(page_id)
+                        .unwrap() // UNWRAP: index indicates that data exists.
+                }
+            })
+            .cloned()
+    }
+
+    /// Get a value change by ID.
+    ///
+    /// `None` indicates that the value has not changed in the overlay, not that the value doesn't
+    /// exist.
+    pub(super) fn value(&self, key: &KeyPath) -> Option<ValueChange> {
+        self.parent
+            .as_ref()
+            .and_then(|parent| parent.index.values.get(key))
+            .and_then(|seqn| seqn.checked_sub(self.min_seqn))
+            .map(|seqn_diff| {
+                if seqn_diff == 0 {
+                    // UNWRAP: parent existence checked above
+                    // UNWRAP: index indicates that data exists.
+                    self.parent.as_ref().unwrap().data.values.get(key).unwrap()
+                } else {
+                    // UNWRAP: index indicates that data exists.
+                    self.ancestor_data[seqn_diff as usize - 1]
+                        .values
+                        .get(key)
+                        .unwrap()
+                }
+            })
+            .cloned()
+    }
+
+    /// Finish this overlay and transform it into a frozen [`Overlay`].
+    pub(super) fn finish(
+        self,
+        page_changes: HashMap<PageId, (Page, PageDiff)>,
+        value_changes: HashMap<KeyPath, ValueChange>,
+    ) -> Overlay {
+        let new_seqn = self.parent.as_ref().map_or(0, |p| p.seqn + 1);
+
+        // rebuild the index, including the new stuff, and excluding stuff from dead overlays.
+        let mut index = self
+            .parent
+            .as_ref()
+            .map_or_else(Default::default, |p| p.index.clone());
+        index.prune_below(self.min_seqn);
+
+        index.insert_pages(new_seqn, page_changes.keys().cloned());
+        index.insert_values(new_seqn, value_changes.keys().cloned());
+
+        let ancestor_data = self
+            .parent
+            .map(|parent| {
+                std::iter::once(Arc::downgrade(&parent.data))
+                    .chain(self.ancestor_data.into_iter().map(|d| Arc::downgrade(&d)))
+                    .collect()
+            })
+            .unwrap_or_default();
+
+        Overlay {
+            inner: Arc::new(OverlayInner {
+                index,
+                data: Arc::new(Data {
+                    pages: page_changes,
+                    values: value_changes,
+                }),
+                seqn: new_seqn,
+                ancestor_data,
+            }),
+        }
+    }
+}