diff --git a/src/maps.rs b/src/maps.rs
index f79d4a8..161fca5 100644
--- a/src/maps.rs
+++ b/src/maps.rs
@@ -3,25 +3,18 @@ use std::ops::{Index, IndexMut};
 use crate::traits::GraphTopology;
 
 pub struct VertexMap<V: Copy, T: Clone> {
-    data: Vec<T>,
-    default: T,
-    to_index: fn(V) -> usize,
+    inner: EntityMap<V, T>,
 }
 
 impl<V: Copy, T: Clone> VertexMap<V, T> {
     pub fn new(default: T, to_index: fn(V) -> usize, capacity: usize) -> Self {
         Self {
-            data: vec![default.clone(); capacity],
-            default,
-            to_index,
+            inner: EntityMap::new(default, to_index, capacity),
         }
     }
 
     pub fn sync<G: GraphTopology<Vertex = V>>(&mut self, graph: &G) {
-        let capacity = graph.vertex_capacity();
-        if capacity > self.data.len() {
-            self.data.resize(capacity, self.default.clone());
-        }
+        self.inner.resize(graph.vertex_capacity());
     }
 }
 
@@ -29,7 +22,73 @@ impl<V: Copy, T: Clone> Index<V> for VertexMap<V, T> {
     type Output = T;
 
     fn index(&self, v: V) -> &T {
-        let i = (self.to_index)(v);
+        &self.inner[v]
+    }
+}
+
+impl<V: Copy, T: Clone> IndexMut<V> for VertexMap<V, T> {
+    fn index_mut(&mut self, v: V) -> &mut T {
+        &mut self.inner[v]
+    }
+}
+
+pub struct EdgeMap<E: Copy, T: Clone> {
+    inner: EntityMap<E, T>,
+}
+
+impl<E: Copy, T: Clone> EdgeMap<E, T> {
+    pub fn new(default: T, to_index: fn(E) -> usize, capacity: usize) -> Self {
+        Self {
+            inner: EntityMap::new(default, to_index, capacity),
+        }
+    }
+
+    pub fn sync<G: GraphTopology<Edge = E>>(&mut self, graph: &G) {
+        self.inner.resize(graph.edge_capacity());
+    }
+}
+
+impl<E: Copy, T: Clone> Index<E> for EdgeMap<E, T> {
+    type Output = T;
+
+    fn index(&self, e: E) -> &T {
+        &self.inner[e]
+    }
+}
+
+impl<E: Copy, T: Clone> IndexMut<E> for EdgeMap<E, T> {
+    fn index_mut(&mut self, e: E) -> &mut T {
+        &mut self.inner[e]
+    }
+}
+
+struct EntityMap<E: Copy, T: Clone> {
+    data: Vec<T>,
+    default: T,
+    to_index: fn(E) -> usize,
+}
+
+impl<E: Copy, T: Clone> EntityMap<E, T> {
+    pub fn new(default: T, to_index: fn(E) -> usize, capacity: usize) -> Self {
+        Self {
+            data: vec![default.clone(); capacity],
+            default,
+            to_index,
+        }
+    }
+
+    pub fn resize(&mut self, capacity: usize) {
+        if capacity > self.data.len() {
+            self.data.resize(capacity, self.default.clone());
+        }
+    }
+}
+
+impl<E: Copy, T: Clone> Index<E> for EntityMap<E, T> {
+    type Output = T;
+
+    fn index(&self, e: E) -> &T {
+        let i = (self.to_index)(e);
         if i < self.data.len() {
             &self.data[i]
         } else {
@@ -38,9 +97,9 @@ impl<V: Copy, T: Clone> Index<V> for VertexMap<V, T> {
     }
 }
 
-impl<V: Copy, T: Clone> IndexMut<V> for VertexMap<V, T> {
-    fn index_mut(&mut self, v: V) -> &mut T {
-        let i = (self.to_index)(v);
+impl<E: Copy, T: Clone> IndexMut<E> for EntityMap<E, T> {
+    fn index_mut(&mut self, e: E) -> &mut T {
+        let i = (self.to_index)(e);
         if i >= self.data.len() {
             self.data.resize(i + 1, self.default.clone());
         }
@@ -49,7 +108,7 @@ impl<V: Copy, T: Clone> IndexMut<V> for VertexMap<V, T> {
 }
 
 #[cfg(test)]
-mod append_graph_tests {
+mod append_graph_vertex_map_tests {
     use crate::models::append_graph::AppendGraph;
     use crate::traits::GraphTopology;
 
@@ -57,10 +116,27 @@ mod append_graph_tests {
 }
 
 #[cfg(test)]
-mod graph_tests {
+mod append_graph_edge_map_tests {
+    use crate::models::append_graph::AppendGraph;
+    use crate::traits::GraphTopology;
+
+    crate::edge_map_tests!(AppendGraph);
+}
+
+#[cfg(test)]
+mod graph_vertex_map_tests {
     use crate::models::graph::Graph;
     use crate::traits::{GraphTopology, GraphTopologyDeletion};
 
     crate::vertex_map_tests!(Graph);
     crate::vertex_map_deletion_tests!(Graph);
 }
+
+#[cfg(test)]
+mod graph_edge_map_tests {
+    use crate::models::graph::Graph;
+    use crate::traits::{GraphTopology, GraphTopologyDeletion};
+
+    crate::edge_map_tests!(Graph);
+    crate::edge_map_deletion_tests!(Graph);
+}
diff --git a/src/models/append_graph.rs b/src/models/append_graph.rs
index 816e6af..cd4d92a 100644
--- a/src/models/append_graph.rs
+++ b/src/models/append_graph.rs
@@ -1,4 +1,4 @@
-use crate::maps::VertexMap;
+use crate::maps::{EdgeMap, VertexMap};
 use crate::traits::GraphTopology;
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
@@ -92,6 +92,14 @@ impl GraphTopology for AppendGraph {
         self.incidences.len() / 2
     }
 
+    fn edge_capacity(&self) -> usize {
+        self.incidences.len() / 2
+    }
+
+    fn edge_map<T: Clone>(&self, default: T) -> EdgeMap<Self::Edge, T> {
+        EdgeMap::new(default, |e| e.0 / 2, self.edge_capacity())
+    }
+
     fn degree(&self, v: Self::Vertex) -> usize {
         self.vertices[v.0].incidence_count
     }
diff --git a/src/models/graph.rs b/src/models/graph.rs
index 8970e24..41c7f95 100644
--- a/src/models/graph.rs
+++ b/src/models/graph.rs
@@ -1,6 +1,6 @@
 use typed_generational_arena::{Arena, Index};
 
-use crate::maps::VertexMap;
+use crate::maps::{EdgeMap, VertexMap};
 use crate::traits::{GraphTopology, GraphTopologyDeletion};
 
 type Vertex = Index<VertexIncidenceHeader, usize, usize>;
@@ -171,6 +171,14 @@ impl GraphTopology for Graph {
         self.incidences.len() / 2
     }
 
+    fn edge_capacity(&self) -> usize {
+        self.incidences.capacity() / 2
+    }
+
+    fn edge_map<T: Clone>(&self, default: T) -> EdgeMap<Self::Edge, T> {
+        EdgeMap::new(default, |e| e.arr_idx() / 2, self.edge_capacity())
+    }
+
     fn degree(&self, v: Self::Vertex) -> usize {
         self.vertices[v].incidence_count
     }
diff --git a/src/testing.rs b/src/testing.rs
index 7975b1f..6dccf6c 100644
--- a/src/testing.rs
+++ b/src/testing.rs
@@ -1,2 +1,2 @@
 pub(crate) mod graph_topology_testing;
-pub(crate) mod vertex_map_testing;
+pub(crate) mod maps_testing;
diff --git a/src/testing/maps_testing.rs b/src/testing/maps_testing.rs
new file mode 100644
index 0000000..bc73c6f
--- /dev/null
+++ b/src/testing/maps_testing.rs
@@ -0,0 +1,261 @@
+#[cfg(test)]
+#[macro_export]
+macro_rules! vertex_map_tests {
+    ($T:ty) => {
+        #[test]
+        fn initial_values_are_default() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let v2 = graph.add_vertex();
+            let map = graph.vertex_map(42);
+            assert_eq!(map[v1], 42);
+            assert_eq!(map[v2], 42);
+        }
+
+        #[test]
+        fn write_and_read() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let v2 = graph.add_vertex();
+            let mut map = graph.vertex_map(0);
+            map[v1] = 7;
+            assert_eq!(map[v1], 7);
+            assert_eq!(map[v2], 0);
+        }
+
+        #[test]
+        fn lazy_growth_on_read() {
+            let mut graph = <$T>::new();
+            graph.add_vertex();
+            let map = graph.vertex_map(99);
+            let v = graph.add_vertex();
+            assert_eq!(map[v], 99);
+        }
+
+        #[test]
+        fn lazy_growth_on_write() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let mut map = graph.vertex_map(0);
+            let v2 = graph.add_vertex();
+            map[v2] = 7;
+            assert_eq!(map[v1], 0);
+            assert_eq!(map[v2], 7);
+        }
+
+        #[test]
+        fn sync_expands_to_new_vertices() {
+            let mut graph = <$T>::new();
+            graph.add_vertex();
+            let mut map = graph.vertex_map(42);
+            let initial_len = map.inner.data.len();
+            while graph.vertex_capacity() <= initial_len {
+                graph.add_vertex();
+            }
+            assert!(
+                map.inner.data.len() < graph.vertex_capacity(),
+                "precondition: map is stale before sync"
+            );
+            map.sync(&graph);
+            assert_eq!(map.inner.data.len(), graph.vertex_capacity());
+        }
+
+        #[test]
+        fn sync_does_not_overwrite_existing_values() {
+            let mut graph = <$T>::new();
+            let v = graph.add_vertex();
+            let mut map = graph.vertex_map(0);
+            map[v] = 5;
+            graph.add_vertex();
+            map.sync(&graph);
+            assert_eq!(map[v], 5);
+        }
+    };
+}
+
+#[cfg(test)]
+#[macro_export]
+macro_rules! vertex_map_deletion_tests {
+    ($T:ty) => {
+        #[test]
+        fn surviving_vertex_readable_after_delete() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let v2 = graph.add_vertex();
+            let mut map = graph.vertex_map(0);
+            map[v1] = 1;
+            map[v2] = 2;
+            graph.delete_vertex(v2);
+            assert_eq!(map[v1], 1);
+        }
+
+        #[test]
+        fn capacity_does_not_shrink_after_delete() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let v2 = graph.add_vertex();
+            let mut map = graph.vertex_map(0);
+            map[v1] = 5;
+            let capacity_before = graph.vertex_capacity();
+            graph.delete_vertex(v2);
+            map.sync(&graph);
+            assert_eq!(map.inner.data.len(), capacity_before);
+            assert_eq!(map[v1], 5);
+        }
+
+        #[test]
+        fn reused_slot_returns_old_value() {
+            let mut graph = <$T>::new();
+            graph.add_vertex();
+            let v1 = graph.add_vertex();
+            let mut map = graph.vertex_map(0);
+            map[v1] = 99;
+            graph.delete_vertex(v1);
+            let v2 = graph.add_vertex();
+            // VertexMap uses raw indices, not vertex identity. A new vertex v2 reusing the slot
+            // of previously deleted v1 sees the old value. Callers must reinitialize stale slots
+            // after deletion.
+            assert_eq!(map[v2], 99);
+        }
+    };
+}
+
+#[cfg(test)]
+#[macro_export]
+macro_rules! edge_map_tests {
+    ($T:ty) => {
+        #[test]
+        fn initial_values_are_default() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let v2 = graph.add_vertex();
+            let e1 = graph.add_edge(v1, v2);
+            let e2 = graph.add_edge(v1, v2);
+            let map = graph.edge_map(42);
+            assert_eq!(map[e1], 42);
+            assert_eq!(map[e2], 42);
+        }
+
+        #[test]
+        fn write_and_read() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let v2 = graph.add_vertex();
+            let e1 = graph.add_edge(v1, v2);
+            let e2 = graph.add_edge(v1, v2);
+            let mut map = graph.edge_map(0);
+            map[e1] = 7;
+            assert_eq!(map[e1], 7);
+            assert_eq!(map[e2], 0);
+        }
+
+        #[test]
+        fn lazy_growth_on_read() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let v2 = graph.add_vertex();
+            graph.add_edge(v1, v2);
+            let map = graph.edge_map(99);
+            let e = graph.add_edge(v1, v2);
+            assert_eq!(map[e], 99);
+        }
+
+        #[test]
+        fn lazy_growth_on_write() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let v2 = graph.add_vertex();
+            let e1 = graph.add_edge(v1, v2);
+            let mut map = graph.edge_map(0);
+            let e2 = graph.add_edge(v1, v2);
+            map[e2] = 7;
+            assert_eq!(map[e1], 0);
+            assert_eq!(map[e2], 7);
+        }
+
+        #[test]
+        fn sync_expands_to_new_edges() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let v2 = graph.add_vertex();
+            graph.add_edge(v1, v2);
+            let mut map = graph.edge_map(42);
+            let initial_len = map.inner.data.len();
+            while graph.edge_capacity() <= initial_len {
+                graph.add_edge(v1, v2);
+            }
+            assert!(
+                map.inner.data.len() < graph.edge_capacity(),
+                "precondition: map is stale before sync"
+            );
+            map.sync(&graph);
+            assert_eq!(map.inner.data.len(), graph.edge_capacity());
+        }
+
+        #[test]
+        fn sync_does_not_overwrite_existing_values() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let v2 = graph.add_vertex();
+            let e = graph.add_edge(v1, v2);
+            let mut map = graph.edge_map(0);
+            map[e] = 5;
+            graph.add_edge(v1, v2);
+            map.sync(&graph);
+            assert_eq!(map[e], 5);
+        }
+    };
+}
+
+#[cfg(test)]
+#[macro_export]
+macro_rules! edge_map_deletion_tests {
+    ($T:ty) => {
+        #[test]
+        fn surviving_edge_readable_after_delete() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let v2 = graph.add_vertex();
+            let e1 = graph.add_edge(v1, v2);
+            let e2 = graph.add_edge(v1, v2);
+            let mut map = graph.edge_map(0);
+            map[e1] = 1;
+            map[e2] = 2;
+            graph.delete_edge(e2);
+            assert_eq!(map[e1], 1);
+        }
+
+        #[test]
+        fn capacity_does_not_shrink_after_delete() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let v2 = graph.add_vertex();
+            let e1 = graph.add_edge(v1, v2);
+            let e2 = graph.add_edge(v1, v2);
+            let mut map = graph.edge_map(0);
+            map[e1] = 5;
+            let capacity_before = graph.edge_capacity();
+            graph.delete_edge(e2);
+            map.sync(&graph);
+            assert_eq!(map.inner.data.len(), capacity_before);
+            assert_eq!(map[e1], 5);
+        }
+
+        #[test]
+        fn reused_slot_returns_old_value() {
+            let mut graph = <$T>::new();
+            let v1 = graph.add_vertex();
+            let v2 = graph.add_vertex();
+            graph.add_edge(v1, v2);
+            let e1 = graph.add_edge(v1, v2);
+            let mut map = graph.edge_map(0);
+            map[e1] = 99;
+            graph.delete_edge(e1);
+            let e2 = graph.add_edge(v1, v2);
+            // EdgeMap uses raw indices, not edge identity. Because to_index uses arr_idx/2,
+            // both halves of a deleted edge pair map to the same index, so a new edge reusing
+            // either slot sees the old value. Callers must reinitialize stale slots after deletion.
+            assert_eq!(map[e2], 99);
+        }
+    };
+}
diff --git a/src/testing/vertex_map_testing.rs b/src/testing/vertex_map_testing.rs
deleted file mode 100644
index e97df88..0000000
--- a/src/testing/vertex_map_testing.rs
+++ /dev/null
@@ -1,120 +0,0 @@
-#[cfg(test)]
-#[macro_export]
-macro_rules! vertex_map_tests {
-    ($T:ty) => {
-        #[test]
-        fn initial_values_are_default() {
-            let mut graph = <$T>::new();
-            let v1 = graph.add_vertex();
-            let v2 = graph.add_vertex();
-            let map = graph.vertex_map(42);
-            assert_eq!(map[v1], 42);
-            assert_eq!(map[v2], 42);
-        }
-
-        #[test]
-        fn write_and_read() {
-            let mut graph = <$T>::new();
-            let v1 = graph.add_vertex();
-            let v2 = graph.add_vertex();
-            let mut map = graph.vertex_map(0);
-            map[v1] = 7;
-            assert_eq!(map[v1], 7);
-            assert_eq!(map[v2], 0);
-        }
-
-        #[test]
-        fn lazy_growth_on_read() {
-            let mut graph = <$T>::new();
-            graph.add_vertex();
-            let map = graph.vertex_map(99);
-            let v = graph.add_vertex();
-            assert_eq!(map[v], 99);
-        }
-
-        #[test]
-        fn lazy_growth_on_write() {
-            let mut graph = <$T>::new();
-            let v1 = graph.add_vertex();
-            let mut map = graph.vertex_map(0);
-            let v2 = graph.add_vertex();
-            map[v2] = 7;
-            assert_eq!(map[v1], 0);
-            assert_eq!(map[v2], 7);
-        }
-
-        #[test]
-        fn sync_expands_to_new_vertices() {
-            let mut graph = <$T>::new();
-            graph.add_vertex();
-            let mut map = graph.vertex_map(42);
-            let initial_len = map.data.len();
-            while graph.vertex_capacity() <= initial_len {
-                graph.add_vertex();
-            }
-            assert!(
-                map.data.len() < graph.vertex_capacity(),
-                "precondition: map is stale before sync"
-            );
-            map.sync(&graph);
-            assert_eq!(map.data.len(), graph.vertex_capacity());
-        }
-
-        #[test]
-        fn sync_does_not_overwrite_existing_values() {
-            let mut graph = <$T>::new();
-            let v = graph.add_vertex();
-            let mut map = graph.vertex_map(0);
-            map[v] = 5;
-            graph.add_vertex();
-            map.sync(&graph);
-            assert_eq!(map[v], 5);
-        }
-    };
-}
-
-#[cfg(test)]
-#[macro_export]
-macro_rules! vertex_map_deletion_tests {
-    ($T:ty) => {
-        #[test]
-        fn surviving_vertex_readable_after_delete() {
-            let mut graph = <$T>::new();
-            let v1 = graph.add_vertex();
-            let v2 = graph.add_vertex();
-            let mut map = graph.vertex_map(0);
-            map[v1] = 1;
-            map[v2] = 2;
-            graph.delete_vertex(v2);
-            assert_eq!(map[v1], 1);
-        }
-
-        #[test]
-        fn capacity_does_not_shrink_after_delete() {
-            let mut graph = <$T>::new();
-            let v1 = graph.add_vertex();
-            let v2 = graph.add_vertex();
-            let mut map = graph.vertex_map(0);
-            let capacity_before = graph.vertex_capacity();
-            graph.delete_vertex(v2);
-            map.sync(&graph);
-            assert_eq!(map.data.len(), capacity_before);
-            assert_eq!(map[v1], 0);
-        }
-
-        #[test]
-        fn reused_slot_returns_old_value() {
-            let mut graph = <$T>::new();
-            graph.add_vertex();
-            let v1 = graph.add_vertex();
-            let mut map = graph.vertex_map(0);
-            map[v1] = 99;
-            graph.delete_vertex(v1);
-            let v2 = graph.add_vertex();
-            // VertexMap uses raw indices, not vertex identity. A new vertex v2 reusing the slot
-            // of previously deleted v1 sees the old value. Callers must reinitialize stale slots
-            // after deletion.
-            assert_eq!(map[v2], 99);
-        }
-    };
-}
diff --git a/src/traits.rs b/src/traits.rs
index 16c7129..9964ad0 100644
--- a/src/traits.rs
+++ b/src/traits.rs
@@ -1,4 +1,4 @@
-use crate::maps::VertexMap;
+use crate::maps::{EdgeMap, VertexMap};
 
 // TODO: Add functions to reserve memory for vertices and edges.
 // TODO: Add iterator of incident edges for a vertex.
@@ -12,6 +12,8 @@ pub trait GraphTopology {
     fn vertex_capacity(&self) -> usize;
     fn vertex_map<T: Clone>(&self, default: T) -> VertexMap<Self::Vertex, T>;
     fn edge_count(&self) -> usize;
+    fn edge_capacity(&self) -> usize;
+    fn edge_map<T: Clone>(&self, default: T) -> EdgeMap<Self::Edge, T>;
     fn degree(&self, v: Self::Vertex) -> usize;
     fn are_adjacent(&self, v1: Self::Vertex, v2: Self::Vertex) -> bool;
     fn vertices(&self) -> impl Iterator<Item = Self::Vertex>;