diff --git a/README.txt b/README.txt
index 0e8beab..ca6399d 100644
--- a/README.txt
+++ b/README.txt
@@ -40,7 +40,8 @@ Release:
         + Dynamic svg cursor to represent the brush
         - Eraser
         - Line drawing
-        - Undo/redo
+        + Undo
+        - Redo
     * Polish
         + Use typedvector where appropriate
         - Show what's happening while the desk is loading (downloading, processing, uploading to gpu)
diff --git a/client/bvh.js b/client/bvh.js
index 73ebaf9..d330e98 100644
--- a/client/bvh.js
+++ b/client/bvh.js
@@ -89,9 +89,11 @@ function bvh_find_best_sibling(bvh, leaf_index) {
     return best_index;
 }
 
-function bvh_add_stroke(bvh, index, stroke) {
+function bvh_add_stroke(state, bvh, index, stroke) {
     const leaf_index = bvh_make_leaf(bvh, index, stroke);
 
+    stroke.bvh_node = leaf_index;
+
     if (bvh.nodes.length === 1) {
         bvh.root = leaf_index;
         return;
@@ -157,7 +159,22 @@ function bvh_add_stroke(bvh, index, stroke) {
     }
 }
 
-function bvh_intersect_quad(bvh, quad, result_buffer) {
+function bvh_delete_stroke(state, stroke) {
+    let node = state.bvh.nodes[stroke.bvh_node];
+    
+    while (node.parent_index !== null) {
+        if (node.is_fullnode) {
+            let index_index = node.stroke_indices.data.indexOf(stroke.index);
+            node.stroke_indices.data[index_index] = node.stroke_indices.data[node.stroke_indices.size - 1];
+            tv_pop(node.stroke_indices);
+            break;
+        }
+
+        node = state.bvh.nodes[node.parent_index];
+    }
+}
+
+function bvh_intersect_quad(state, bvh, quad, result_buffer) {
     if (bvh.root === null) {
         return;
     }
@@ -181,7 +198,9 @@ function bvh_intersect_quad(bvh, quad, result_buffer) {
         }
 
         if (node.is_leaf) {
-            tv_add(result_buffer, node.stroke_index);
+            if (state.events[node.stroke_index].deleted !== true) {
+                tv_add(result_buffer, node.stroke_index);
+            }
         } else {
             tv_add(bvh.traverse_stack, node.child1);
             tv_add(bvh.traverse_stack, node.child2);
@@ -211,7 +230,7 @@ function bvh_clip(state, context) {
         'y2': screen_bottomright.y
     };
 
-    bvh_intersect_quad(state.bvh, screen, context.clipped_indices);
+    bvh_intersect_quad(state, state.bvh, screen, context.clipped_indices);
 
     tv_data(context.clipped_indices).sort(); // we need to draw back to front still!
 }
@@ -241,7 +260,7 @@ function bvh_point(state, p) {
             const ys = state.wasm.buffers['ys'].tv.data.subarray(stroke.coords_from, stroke.coords_to);
             const pressures = state.wasm.buffers['pressures'].tv.data.subarray(stroke.coords_from, stroke.coords_to);
 
-            if (point_in_stroke(p, xs, ys, pressures, stroke.width)) {
+            if (stroke.deleted !== true && point_in_stroke(p, xs, ys, pressures, stroke.width)) {
                 indices.push(node.stroke_index);
             }
         } else {
@@ -258,7 +277,7 @@ function bvh_point(state, p) {
     return null;
 }
 
-function bvh_construct_rec(bvh, vertical, strokes, depth) {
+function bvh_construct_rec(state, bvh, vertical, strokes, depth) {
     if (strokes.length > 1) {
         // internal
         let sorted_strokes;
@@ -272,8 +291,8 @@ function bvh_construct_rec(bvh, vertical, strokes, depth) {
         const node_index = bvh_make_internal(bvh);
         const left_of_split_count = Math.floor(strokes.length / 2);
 
-        const child1 = bvh_construct_rec(bvh, !vertical, sorted_strokes.slice(0, left_of_split_count), depth + 1);
-        const child2 = bvh_construct_rec(bvh, !vertical, sorted_strokes.slice(left_of_split_count, sorted_strokes.length), depth + 1);
+        const child1 = bvh_construct_rec(state, bvh, !vertical, sorted_strokes.slice(0, left_of_split_count), depth + 1);
+        const child2 = bvh_construct_rec(state, bvh, !vertical, sorted_strokes.slice(left_of_split_count, sorted_strokes.length), depth + 1);
 
         bvh.nodes[child1].parent_index = node_index; 
         bvh.nodes[child2].parent_index = node_index;
@@ -296,12 +315,15 @@ function bvh_construct_rec(bvh, vertical, strokes, depth) {
         return node_index;
     } else {
         // leaf
-        return bvh_make_leaf(bvh, strokes[0].index, strokes[0]);
+        const leaf_index = bvh_make_leaf(bvh, strokes[0].index, strokes[0]);
+        state.events[strokes[0].index].bvh_node = leaf_index;
+        return leaf_index;
     }
 }
 
 function bvh_construct(state) {
-    if (state.events.length > 0) {
-        state.bvh.root = bvh_construct_rec(state.bvh, true, state.events, 0); 
+    const strokes = state.events.filter(e => e.type === EVENT.STROKE && e.deleted !== true);
+    if (strokes.length > 0) {
+        state.bvh.root = bvh_construct_rec(state, state.bvh, true, strokes, 0); 
     }
 }
diff --git a/client/client_recv.js b/client/client_recv.js
index 93ad814..262c165 100644
--- a/client/client_recv.js
+++ b/client/client_recv.js
@@ -245,7 +245,17 @@ function handle_event(state, context, event, options = {}) {
                 break;
             }
 
-            wasm_ensure_by(state, 1, event.coords.length);
+            let last_stroke = null;
+
+            for (let i = state.events.length - 1; i >= 0; --i) {
+                if (state.events[i].type === EVENT.STROKE) {
+                    last_stroke = state.events[i];
+                    break;
+                }
+            }
+
+            const index_difference = state.events.length - (last_stroke === null ? 0 : last_stroke.index);
+            wasm_ensure_by(state, index_difference, event.coords.length);
            
             const pressures = state.wasm.buffers['pressures'];
             const xs = state.wasm.buffers['xs'];
@@ -254,6 +264,13 @@ function handle_event(state, context, event, options = {}) {
             event.coords_from = xs.tv.size;
             event.coords_to = xs.tv.size + point_count;
 
+            for (let i = 0; i < index_difference - 1; ++i) {
+                // Create empty records for all non-stroke events that happened since the last stroke
+                tv_add(state.wasm.buffers['coords_from'].tv, xs.tv.size);
+                state.wasm.buffers['coords_from'].used += 4; // 4 bytes, not 4 ints
+            }
+
+            // Create actual records for this stroke
             tv_add(state.wasm.buffers['coords_from'].tv, xs.tv.size + point_count);
             state.wasm.buffers['coords_from'].used += 4; // 4 bytes, not 4 ints
 
@@ -285,44 +302,28 @@ function handle_event(state, context, event, options = {}) {
         }
 
         case EVENT.UNDO: {
-            need_draw = true;
-            console.error('todo');
-            // for (let i = state.events.length - 1; i >=0; --i) {
-            //     const other_event = state.events[i];
-
-            //     // Users can only undo their own, undeleted (not already undone) events
-            //     if (other_event.user_id === event.user_id && !other_event.deleted) {
-            //         if (other_event.type === EVENT.STROKE) {
-            //             other_event.deleted = true;
-            //             const stats = stroke_stats(other_event.points, state.cursor.width);
-            //             redraw_region(stats.bbox);
-            //             break;
-            //         } else if (other_event.type === EVENT.IMAGE) {
-            //             other_event.deleted = true;
-            //             const item = document.querySelector(`img[data-image-id="${other_event.image_id}"]`);
-            //             if (item) item.remove();
-            //             break;
-            //         } else if (other_event.type === EVENT.ERASER) {
-            //             other_event.deleted = true;
-            //             const erased = find_stroke_backwards(other_event.stroke_id);
-            //             if (erased) {
-            //                 erased.deleted = false;
-            //                 const stats = stroke_stats(erased.points, state.cursor.width);
-            //                 redraw_region(stats.bbox);
-            //             }
-            //             break;
-            //         } else if (other_event.type === EVENT.IMAGE_MOVE) {
-            //             const item = document.querySelector(`img[data-image-id="${other_event.image_id}"]`);
-                        
-            //             const ix = state.images[other_event.image_id].x -= other_event.x;
-            //             const iy = state.images[other_event.image_id].y -= other_event.y;
-
-            //             item.style.transform = `translate(${ix}px, ${iy}px)`;
-
-            //             break;
-            //         }
-            //     }
-            // }
+            geometry_add_dummy_stroke(context);
+
+            for (let i = state.events.length - 1; i >=0; --i) {
+                const other_event = state.events[i];
+
+                // Users can only undo their own, undeleted (not already undone) events
+                if (other_event.user_id === event.user_id && !other_event.deleted) {
+                    if (other_event.type === EVENT.STROKE) {
+                        other_event.deleted = true;
+                        if (other_event.bvh_node) {
+                            bvh_delete_stroke(state, other_event);
+                        }
+                        need_draw = true;
+                        break;
+                    } else if (other_event.type === EVENT.UNDO) {
+                        // do not undo an undo, we are not Notepad
+                    } else {
+                        console.error('cant undo event type', other_event.type);
+                        break;
+                    }
+                }
+            }
 
             break;
         }
diff --git a/client/client_send.js b/client/client_send.js
index 4877fd8..e595d8e 100644
--- a/client/client_send.js
+++ b/client/client_send.js
@@ -369,3 +369,9 @@ function movecanvas_event(state) {
         'zoom': state.canvas.zoom,
     };
 }
+
+function undo_event(state) {
+    return {
+        'type': EVENT.UNDO,
+    };
+}
diff --git a/client/index.js b/client/index.js
index 5333281..69d77e0 100644
--- a/client/index.js
+++ b/client/index.js
@@ -15,7 +15,7 @@ const config = {
     debug_print: false,
     zoom_delta: 0.05,
     min_zoom_level: -250,
-    max_zoom_level: 40,
+    max_zoom_level: 100,
     initial_offline_timeout: 1000,
     default_color: 0x00,
     default_width: 8,
diff --git a/client/tools.js b/client/tools.js
index 09be8ea..5b28778 100644
--- a/client/tools.js
+++ b/client/tools.js
@@ -2,6 +2,7 @@ function switch_tool(state, item) {
     const tool = item.getAttribute('data-tool');
 
     if (tool === 'undo') {
+        queue_event(state, undo_event(state));
         return;
     }
 
diff --git a/client/webgl_geometry.js b/client/webgl_geometry.js
index e881107..a9be331 100644
--- a/client/webgl_geometry.js
+++ b/client/webgl_geometry.js
@@ -48,6 +48,14 @@ async function geometry_write_instances(state, context, callback) {
     return segment_count;
 }
 
+function geometry_add_dummy_stroke(context) {
+    context.stroke_data = ser_ensure_by(context.stroke_data, config.bytes_per_stroke);
+    ser_u16(context.stroke_data, 0);
+    ser_u16(context.stroke_data, 0);
+    ser_u16(context.stroke_data, 0);
+    ser_u16(context.stroke_data, 0);
+}
+
 function geometry_add_stroke(state, context, stroke, stroke_index, skip_bvh = false) {
     if (!state.online || !stroke || stroke.coords_to - stroke.coords_from === 0) return;
 
@@ -66,7 +74,7 @@ function geometry_add_stroke(state, context, stroke, stroke_index, skip_bvh = fa
     ser_u16(context.stroke_data, b);
     ser_u16(context.stroke_data, stroke.width);
 
-    if (!skip_bvh) bvh_add_stroke(state.bvh, stroke_index, stroke);
+    if (!skip_bvh) bvh_add_stroke(state, state.bvh, stroke_index, stroke);
 }
 
 function geometry_delete_stroke(state, context, stroke_index) {
diff --git a/client/webgl_listeners.js b/client/webgl_listeners.js
index b00910c..56dc268 100644
--- a/client/webgl_listeners.js
+++ b/client/webgl_listeners.js
@@ -146,7 +146,11 @@ function keydown(e, state, context) {
     } else if (e.code === 'KeyD') {
         document.querySelector('.debug-window').classList.toggle('dhide');
     } else if (e.code === 'KeyZ') {
-        state.zoomdown = true;
+        if (e.ctrlKey) {
+            queue_event(state, undo_event(state));
+        } else {
+            state.zoomdown = true;
+        }
     }
 }