diff --git a/index.html b/index.html
index d6663c2..2afceb9 100644
--- a/index.html
+++ b/index.html
@@ -18,6 +18,7 @@
       <button disabled id="model_name"></button>
       <button id="select_state_button">Select State</button>
       <button disabled id="state_name"></button>
+      <button id="edit_state_button">Edit State</button>
       <button id="up_button">UP</button>
       <button id="down_button">DOWN</button>
       <button id="left_button">LEFT</button>
diff --git a/src/main.js b/src/main.js
index e772d40..ff91202 100644
--- a/src/main.js
+++ b/src/main.js
@@ -101,6 +101,34 @@ const clear_visualization = () => {
   canvas.height = vh - 43;
 
   context.clearRect(0, 0, canvas.width, canvas.height);
+
+  draw_grid();
+};
+
+const draw_grid = () => {
+  const canvas = document.getElementById("model_canvas");
+  const context = canvas.getContext("2d");
+
+  const [vw, vh] = get_viewport_dims();
+  canvas.width = vw / 2 - 9.5;
+  canvas.height = vh - 43;
+
+  const square_width = canvas.width / current_state.width;
+  const square_height = canvas.height / current_state.height;
+
+  // Horizontal lines
+  for (let y = 1; y < current_state.height; y += 1) {
+    context.moveTo(0, y * square_height);
+    context.lineTo(canvas.width, y * square_height);
+    context.stroke();
+  }
+
+  // Vertical lines
+  for (let x = 1; x < current_state.width; x += 1) {
+    context.moveTo(x * square_width, 0);
+    context.lineTo(x * square_width, canvas.height);
+    context.stroke();
+  }
 };
 
 //
@@ -138,6 +166,76 @@ const node_click_view_state = (node, graph) => {
 // Set up model event-handlers
 //
 
+let editing = false;
+let startpos = null;
+
+document
+  .getElementById("model_canvas")
+  .addEventListener("mousedown", (event) => {
+    if (model.select(current_state, event.offsetX, event.offsetY) === null) {
+      startpos = [event.offsetX, event.offsetY];
+    }
+  });
+
+document.getElementById("model_canvas").addEventListener("mouseup", (event) => {
+  if (startpos === null) {
+    return;
+  }
+
+  let endpos = [event.offsetX, event.offsetY];
+
+  const canvas = document.getElementById("model_canvas");
+
+  const square_width = canvas.width / current_state.width;
+  const square_height = canvas.height / current_state.height;
+
+  // Coordinates
+  const startx = Math.floor(startpos[0] / square_width);
+  const starty = Math.floor(startpos[1] / square_height);
+  const endx = Math.floor(endpos[0] / square_width);
+  const endy = Math.floor(endpos[1] / square_height);
+
+  // Check that the block to be added doesn't collide with anything
+  for (let ix = startx; ix <= endx; ix++) {
+    for (let iy = starty; iy <= endy; iy++) {
+      if (
+        model.select(current_state, ix * square_width, iy * square_height) !==
+        null
+      ) {
+        startpos = null;
+        return;
+      }
+    }
+  }
+
+  // Add block
+  let new_block = model.add_block(
+    current_state,
+    Math.min(startx, endx),
+    Math.min(starty, endy),
+    Math.max(startx, endx),
+    Math.max(starty, endy),
+  );
+
+  if (new_block === null) {
+    startpos = null;
+    return;
+  }
+
+  // Clear graph + visualize
+  selected_element = new_block;
+  clear_visualization();
+  model.visualize(current_state);
+  model.highlight(current_state, selected_element);
+
+  // Generate graph for new state
+  clear_graph();
+  model.generate(states, initial_state, graph, null);
+  reset_graph_view(graph);
+
+  startpos = null;
+});
+
 document.getElementById("model_canvas").addEventListener("click", (event) => {
   const element = model.select(current_state, event.offsetX, event.offsetY);
 
@@ -218,6 +316,7 @@ document.getElementById("select_model_button").addEventListener("click", () => {
   clear_graph();
   clear_visualization();
   model.visualize(initial_state);
+  editing = false;
   document.getElementById("model_name").innerHTML = model.name;
   document.getElementById("state_name").innerHTML = initial_state.name;
 });
@@ -232,12 +331,29 @@ document.getElementById("select_state_button").addEventListener("click", () => {
   states.add(initial_state);
   clear_graph();
   clear_visualization();
-  // graph = null;
-  // graph = generate_graph(data, node_click_view_state);
   model.visualize(initial_state);
+  editing = false;
   document.getElementById("state_name").innerHTML = initial_state.name;
 });
 
+document.getElementById("edit_state_button").addEventListener("click", () => {
+  if (editing === false) {
+    current_initial_state = 0;
+    initial_state = model.empty_state;
+    current_state = initial_state;
+    selected_element = null;
+    clear_states();
+    states.add(initial_state);
+    clear_graph();
+    clear_visualization();
+    model.visualize(initial_state);
+    editing = true;
+    document.getElementById("state_name").innerHTML = initial_state.name;
+  } else {
+    editing = false;
+  }
+});
+
 document
   .getElementById("generate_graph_button")
   .addEventListener("click", () => {
diff --git a/src/simulator/restricted_sliding_blocks/model.js b/src/simulator/restricted_sliding_blocks/model.js
index 104ba6b..fe160fc 100644
--- a/src/simulator/restricted_sliding_blocks/model.js
+++ b/src/simulator/restricted_sliding_blocks/model.js
@@ -183,6 +183,22 @@ const visualize_path = (from_state, to_state) => {
   // For each state in path: visualize(state)
 };
 
+const add_block = (state, x0, y0, x1, y1) => {
+  let dir;
+  if (x1 - x0 > y1 - y0) {
+    dir = HORIZONTAL;
+  } else if (x1 - x0 < y1 - y0) {
+    dir = VERTICAL;
+  } else {
+    // No square blocks for this model
+    return null;
+  }
+
+  let new_block = [state.board.length, dir, x0, y0, x1, y1];
+  state.board.push(new_block);
+  return new_block;
+};
+
 export const restricted_sliding_blocks_model = {
   name: "Restricted Sliding Blocks",
   generate,
@@ -192,4 +208,11 @@ export const restricted_sliding_blocks_model = {
   initial_states,
   select,
   move,
+  empty_state: {
+    name: "Custom",
+    width: 5,
+    height: 5,
+    board: [],
+  },
+  add_block,
 };
diff --git a/src/simulator/sliding_blocks/model.js b/src/simulator/sliding_blocks/model.js
index f018d5f..10bb7b8 100644
--- a/src/simulator/sliding_blocks/model.js
+++ b/src/simulator/sliding_blocks/model.js
@@ -162,6 +162,12 @@ const visualize_path = (from_state, to_state) => {
   // For each state in path: visualize(state)
 };
 
+const add_block = (state, x0, y0, x1, y1) => {
+  let new_block = [state.board.length, x0, y0, x1, y1];
+  state.board.push(new_block);
+  return new_block;
+};
+
 export const sliding_blocks_model = {
   name: "Sliding Blocks",
   generate,
@@ -171,4 +177,11 @@ export const sliding_blocks_model = {
   initial_states,
   select,
   move,
+  empty_state: {
+    name: "Custom",
+    width: 5,
+    height: 6,
+    board: [],
+  },
+  add_block,
 };