diff --git a/src/AstGen.zig b/src/AstGen.zig
index a137046..a516b67 100644
--- a/src/AstGen.zig
+++ b/src/AstGen.zig
@@ -1134,12 +1134,13 @@ fn assign(gi: *GenIr, scope: *Scope, node: *const Ast.Node) InnerError!void {
     const expr_node = node.data.bin.rhs.?;
     const name_str = try astgen.strFromNode(identifier_node);
 
-    if (scope.lookup(name_str.index)) |decl| {
-        const expr_result = try expr(gi, scope, expr_node);
-        _ = try gi.addBinaryNode(.store, decl.inst_index.toRef(), expr_result);
-        return;
-    }
-    _ = try gi.addStrTok(.decl_ref, name_str.index, node.loc.start);
+    const lhs = if (scope.lookup(name_str.index)) |decl| blk: {
+        break :blk decl.inst_index.toRef();
+    } else blk: {
+        break :blk try gi.addStrTok(.decl_ref, name_str.index, node.loc.start);
+    };
+    const rhs = try expr(gi, scope, expr_node);
+    _ = try gi.addBinaryNode(.store, lhs, rhs);
 }
 
 fn assignOp(
diff --git a/src/Story.zig b/src/Story.zig
index a865996..86fc49b 100644
--- a/src/Story.zig
+++ b/src/Story.zig
@@ -24,6 +24,8 @@ choice_selected: ?Choice = null,
 output_buffer: std.ArrayListUnmanaged(OutputCommand) = .empty,
 output_scratch: std.ArrayListUnmanaged(u8) = .empty,
 current_choices: std.ArrayListUnmanaged(Choice) = .empty,
+variable_observers: std.StringHashMapUnmanaged(VariableObserver) = .empty,
+
 globals: std.StringHashMapUnmanaged(Value) = .empty,
 stack: []Value = &.{},
 call_stack: []CallFrame = &.{},
@@ -38,6 +40,17 @@ dump_writer: ?*std.Io.Writer = null,
 
 pub const default_knot_name: [:0]const u8 = "$__main__$";
 
+pub const VariableObserver = struct {
+    callback: Callback,
+    context: Context,
+
+    pub const Callback = *const fn (Value, Context) anyerror!void;
+
+    pub const Context = struct {
+        ptr: *anyopaque,
+    };
+};
+
 pub const Opcode = enum(u8) {
     /// Exit the VM normally.
     exit,
@@ -337,6 +350,7 @@ pub fn deinit(story: *Story) void {
 
     story.arena.deinit();
     story.globals.deinit(gpa);
+    story.variable_observers.deinit(gpa);
     story.current_choices.deinit(gpa);
     story.output_scratch.deinit(gpa);
     story.output_buffer.deinit(gpa);
@@ -491,7 +505,7 @@ const StepSignal = union(enum) {
     choices_ready,
 };
 
-fn step(vm: *Story) !StepSignal {
+fn step(vm: *Story, variables: *VariablesState) !StepSignal {
     assert(vm.call_stack_top > 0);
     assert(vm.can_advance == true);
 
@@ -695,12 +709,13 @@ fn step(vm: *Story) !StepSignal {
             },
             .store_global => {
                 const arg_offset: u8 = @intFromEnum(code[frame.ip + 1]);
-                const global_name = try vm.getConstant(frame, arg_offset);
+                const global_name = try getConstant(vm, frame, arg_offset);
 
-                if (vm.peekStack(0)) |arg| {
-                    try vm.setGlobal(global_name, arg);
-                    _ = vm.popStack();
-                    try vm.pushStack(arg);
+                if (peekStack(vm, 0)) |arg| {
+                    try setGlobal(vm, global_name, arg);
+                    _ = popStack(vm);
+                    try pushStack(vm, arg);
+                    try variables.update(gpa, global_name);
                 } else {
                     return error.InvalidArgument;
                 }
@@ -799,6 +814,27 @@ fn resolveOutputStream(
     return result.toOwnedSlice(gpa);
 }
 
+const VariablesState = struct {
+    changed: std.StringHashMapUnmanaged(void) = .empty,
+
+    pub fn update(
+        state: *VariablesState,
+        gpa: std.mem.Allocator,
+        value: Value,
+    ) !void {
+        switch (value) {
+            .object => |object| switch (object.tag) {
+                .string => {
+                    const str_object: *Object.String = @ptrCast(object);
+                    try state.changed.put(gpa, str_object.toSlice(), undefined);
+                },
+                else => unreachable,
+            },
+            else => unreachable,
+        }
+    }
+};
+
 pub fn advance(story: *Story) !?[]const u8 {
     const arena = story.arena.allocator();
     const output_buffer = &story.output_buffer;
@@ -808,17 +844,26 @@ pub fn advance(story: *Story) !?[]const u8 {
     output_scratch.clearRetainingCapacity();
 
     if (!story.can_advance) return null;
-    while (story.can_advance) {
-        const signal = try story.step();
-        switch (signal) {
-            .exit => {
-                story.is_exited = true;
-                break;
-            },
-            .done, .choices_ready => break,
+    defer story.can_advance = false;
+
+    var variables_state: VariablesState = .{};
+    defer variables_state.changed.deinit(story.gpa);
+
+    const signal = try story.step(&variables_state);
+    switch (signal) {
+        .exit => story.is_exited = true,
+        .done, .choices_ready => {},
+    }
+
+    var state_iter = variables_state.changed.keyIterator();
+    while (state_iter.next()) |key| {
+        if (story.variable_observers.get(key.*)) |observer| {
+            // If there's an observer, we can assume that a global exists.
+            const value = story.globals.get(key.*).?;
+            try observer.callback(value, observer.context);
         }
     }
-    story.can_advance = false;
+
     if (output_buffer.items.len == 0) return null;
     return try resolveOutputStream(story, arena, output_buffer.items[0..]);
 }
@@ -835,6 +880,21 @@ pub fn selectChoiceIndex(story: *Story, index: usize) !void {
     _ = story.arena.reset(.retain_capacity);
 }
 
+pub fn observeVariable(
+    story: *Story,
+    name: []const u8,
+    callback: VariableObserver.Callback,
+    context: VariableObserver.Context,
+) !void {
+    const gpa = story.gpa;
+    if (story.globals.get(name) == null) return error.VariableNotFound;
+
+    try story.variable_observers.put(gpa, name, .{
+        .callback = callback,
+        .context = context,
+    });
+}
+
 pub fn dump(story: *Story, writer: *std.Io.Writer) !void {
     return Dumper.dump(story, writer);
 }
diff --git a/src/Story/runtime_tests.zig b/src/Story/runtime_tests.zig
index 117bf00..252d390 100644
--- a/src/Story/runtime_tests.zig
+++ b/src/Story/runtime_tests.zig
@@ -1,6 +1,7 @@
 const std = @import("std");
 const fatal = std.process.fatal;
 const ink = @import("../root.zig");
+const Story = ink.Story;
 
 test "fixture - variable arithmetic" {
     try testRuntimeFixture("variable-arithmetic");
@@ -256,6 +257,37 @@ test "fixture - I135 (Bools can be coerced)" {
     try testRuntimeFixture("I135");
 }
 
+test "variable observer" {
+    const gpa = std.testing.allocator;
+    var io_w = std.Io.Writer.Allocating.init(gpa);
+    defer io_w.deinit();
+
+    var story = try ink.Story.fromSourceBytes(gpa,
+        \\VAR foo = 1
+        \\~foo = 10
+    , .{
+        .filename = "<STDIN>",
+        .error_writer = &io_w.writer,
+    });
+    defer story.deinit();
+
+    const TestState = struct { score: i64 };
+    var test_state = TestState{ .score = 0 };
+
+    try story.observeVariable("foo", struct {
+        fn observe(
+            value: Story.Value,
+            ctx: Story.VariableObserver.Context,
+        ) anyerror!void {
+            const local_test_state: *TestState = @ptrCast(@alignCast(ctx.ptr));
+            local_test_state.score += value.int;
+        }
+    }.observe, .{ .ptr = &test_state });
+
+    _ = try story.advance();
+    return std.testing.expectEqual(10, test_state.score);
+}
+
 const Options = struct {
     input_reader: *std.Io.Reader,
     error_writer: *std.Io.Writer,
@@ -265,7 +297,7 @@ const Options = struct {
 fn testRunner(gpa: std.mem.Allocator, source_bytes: [:0]const u8, options: Options) !void {
     const io_r = options.input_reader;
     const io_w = options.transcript_writer;
-    var story = try ink.Story.fromSourceBytes(gpa, source_bytes, .{
+    var story = try Story.fromSourceBytes(gpa, source_bytes, .{
         .filename = "<STDIN>",
         .error_writer = options.error_writer,
     });