diff --git a/app.c b/app.c
index 4734e3e..dd0b94a 100644
--- a/app.c
+++ b/app.c
@@ -11,6 +11,7 @@
 #include "render_bundle.h"
 #include "render_component.h"
 #include "mapper_misc.h"
+#include "ui.h"
 #include <stdlib.h>
 #include <stdio.h>
 
@@ -27,6 +28,9 @@ App *app_NewApp() {
 	app->camera   = camera_NewSystem(app);
 	app->particle = particle_NewSystem(app);
 	app->time     = gametime_NewSystem(app);
+	app->ui       = ui_NewSystem(app);
+	ui_RebuildUI(app->ui);
+	ui_PushState(app->ui, ui_Running);
 
 	app->switch_level = NULL;
 	app->timescale    = 1.0;
@@ -57,6 +61,7 @@ void app_DeleteApp(App *app) {
 	camera_DeleteSystem(app->camera);
 	particle_DeleteSystem(app->particle);
 	gametime_DeleteSystem(app->time);
+	ui_DeleteSystem(app->ui);
 
 	free(app);
 }
@@ -72,15 +77,18 @@ void app_Advance(App *app, Duration deltaTime) {
 
 	input_Advance(app->input);
 
+	Duration delta_game = deltaTime;
 	if (1.0 - app->timescale > EPS)
-		deltaTime.microseconds = deltaTime.microseconds * app->timescale;
+		delta_game.microseconds = delta_game.microseconds * app->timescale;
 
 	if (!app->paused) {
-		gametime_Advance(app->time, deltaTime);
-		particle_Advance(app->particle, deltaTime);
-		player_Advance(app->player, deltaTime);
-		physics_Advance(app->physics, deltaTime);
-		entity_Advance(app->entity, deltaTime);
-		camera_Advance(app->camera, deltaTime);
+		gametime_Advance(app->time, delta_game);
+		particle_Advance(app->particle, delta_game);
+		player_Advance(app->player, delta_game);
+		physics_Advance(app->physics, delta_game);
+		entity_Advance(app->entity, delta_game);
+		camera_Advance(app->camera, delta_game);
 	}
+
+	ui_Advance(app->ui, deltaTime);
 }
diff --git a/app.h b/app.h
index f951afc..206affe 100644
--- a/app.h
+++ b/app.h
@@ -7,6 +7,7 @@
 #include "input.h"
 #include "camera.h"
 #include "particle.h"
+#include "ui.h"
 #include "types.h"
 #include "util/vector.h"
 
@@ -25,6 +26,7 @@ typedef struct _App {
 	System_Camera   *camera;
 	System_Particle *particle;
 	System_GameTime *time;
+	System_UI       *ui;
 
 	char    *switch_level;
 	double   timescale;
diff --git a/app_render.cpp b/app_render.cpp
index 9d19e5a..5b436e7 100644
--- a/app_render.cpp
+++ b/app_render.cpp
@@ -7,6 +7,7 @@
 #include "easyx.h"
 #include "render_bundle.h"
 #include "render_component.h"
+#include "ui.h"
 #include "util/tree.h"
 #include "types.h"
 #include "render_util.h"
@@ -21,10 +22,7 @@ TimePoint since = time_Now();
 } // namespace
 
 
-extern "C" {
-
-
-void app_Render(App *app) {
+extern "C" void app_Render(App *app) {
 	render_SetModes(render_ModeDefault, time_Now());
 	setbkcolor(app->clear_color);
 	cleardevice();
@@ -176,11 +174,13 @@ void app_Render(App *app) {
 
 	// If paused, display a text
 	if (app->paused)
-		render_DrawTextEx("Game Paused", box2(SCREEN_WIDTH / 2 - 10, 100, 20, 100), DT_CENTER | DT_NOCLIP);
+		render_DrawTextEx("Game Paused", box2(SCREEN_WIDTH / 2.0 - 10, 100, 20, 100), DT_CENTER | DT_NOCLIP);
 	if (1.0 - app->timescale > EPS) {
 		char buf[128];
 		snprintf(buf, sizeof(buf), "*** TIMESCALE %.2lf ***", app->timescale);
-		render_DrawTextEx(buf, box2(SCREEN_WIDTH / 2 - 10, 50, 20, 100), DT_CENTER | DT_NOCLIP);
+		render_DrawTextEx(buf, box2(SCREEN_WIDTH / 2.0 - 10, 50, 20, 100), DT_CENTER | DT_NOCLIP);
 	}
-}
+
+	// Draw UI
+	ui_Render(app->ui);
 }
diff --git a/input.c b/input.c
index 0f6fd6d..eab3495 100644
--- a/input.c
+++ b/input.c
@@ -1,6 +1,7 @@
 
 #include "input.h"
 #include "app.h"
+#include "ui.h"
 
 #include <stdlib.h>
 #include <stdio.h>
@@ -75,11 +76,15 @@ void input_Advance(System_Input *sys) {
 	}
 
 	if (sys->keys[input_Key_Escape] == JustPressed) {
-		if (!sys->super->paused)
-			fprintf(stderr, "[input_Advance] Pausing\n");
-		else
-			fprintf(stderr, "[input_Advance] Unpausing\n");
-		sys->super->paused = !sys->super->paused;
+		if (ui_CurrentState(sys->super->ui) == ui_Running) {
+			// Pause
+			ui_PushState(sys->super->ui, ui_Paused);
+			sys->super->paused = true;
+		} else if (ui_CurrentState(sys->super->ui) == ui_Paused) {
+			// Unpause
+			ui_PopState(sys->super->ui);
+			sys->super->paused = false;
+		}
 	}
 }
 
diff --git a/types.h b/types.h
index 9b76c29..0008650 100644
--- a/types.h
+++ b/types.h
@@ -11,8 +11,8 @@ extern "C" {
 #endif
 
 
-#define SCREEN_WIDTH  1600
-#define SCREEN_HEIGHT 900
+#define SCREEN_WIDTH  1536
+#define SCREEN_HEIGHT 864
 
 #define WARN(fmt, ...) fprintf(stderr, "[WARN][%s] " fmt "\n", __func__, ##__VA_ARGS__)
 #define INFO(fmt, ...) fprintf(stderr, "[%s] " fmt "\n", __func__, ##__VA_ARGS__)
diff --git a/ui.c b/ui.c
new file mode 100644
index 0000000..0a319dc
--- /dev/null
+++ b/ui.c
@@ -0,0 +1,127 @@
+
+#include "ui.h"
+#include "input.h"
+#include "app.h"
+#include "types.h"
+#include "util/vector.h"
+#include <stdio.h>
+
+
+const char *ui_StateTitle[ui_StateCount] = {
+	"Running",
+	"Pause Menu",
+	"Jack's Escape (ver. 3.141592) - Title",
+	"Select Level",
+	"Options"};
+
+
+static inline double dabs(double x) { return x > 0 ? x : -x; }
+static inline double dmin(double x, double y) { return x < y ? x : y; }
+static inline double dmax(double x, double y) { return x > y ? x : y; }
+
+
+System_UI *ui_NewSystem(App *super) {
+	System_UI *sys = zero_malloc(sizeof(System_UI));
+	sys->super     = super;
+	sys->state     = vector_Create(sizeof(ui_State));
+	for (int i = 0; i < ui_StateCount; i++)
+		sys->parts[i] = vector_Create(sizeof(ui_Part));
+	return sys;
+}
+void ui_DeleteSystem(System_UI *sys) {
+	for (int i = 0; i < ui_StateCount; i++)
+		for (int j = 0; j < vector_Size(sys->parts[i]); j++) {
+			ui_Part *part = (ui_Part *)vector_At(sys->parts[i], j);
+			part->free(sys, part, part->user);
+		}
+	for (int i = 0; i < ui_StateCount; i++)
+		vector_Destroy(sys->parts[i]);
+	vector_Destroy(sys->state);
+	free(sys);
+}
+
+
+void ui_PushPart(System_UI *sys, ui_State layer, ui_Part part) {
+	vector_Push(sys->parts[layer], &part);
+}
+
+
+static inline double fadeto(double from, double to, Duration deltaTime) {
+	if (dabs(from - to) < EPS)
+		return to;
+	else
+		return from + (to - from) * UI_BACKGROUND_FADE_MOVE_SPEED * duration_Seconds(deltaTime);
+}
+
+
+static void _ui_UpdateBgTarget(System_UI *sys) {
+	if (vector_Size(sys->state) == 0) {
+		WARN("state stack is empty");
+		return;
+	}
+
+	ui_State layer = *(ui_State *)vector_Back(sys->state);
+	if (vector_Size(sys->parts[layer]) == 0) {
+		// No UI target, set to (0,0), (0,0)
+		memset(&sys->bg_target, 0, sizeof(sys->bg_target));
+		return;
+	}
+
+	Vec2 min = {.x = SCREEN_WIDTH, .y = SCREEN_HEIGHT}, max = {.x = 0, .y = 0};
+	for (int i = 0; i < vector_Size(sys->parts[layer]); i++) {
+		ui_Part *part = (ui_Part *)vector_At(sys->parts[layer], i);
+		min.x         = dmin(min.x, part->box.lefttop.x);
+		min.y         = dmin(min.y, part->box.lefttop.y);
+		max.x         = dmax(max.x, part->box.lefttop.x + part->box.size.x);
+		max.y         = dmax(max.y, part->box.lefttop.y + part->box.size.y);
+	}
+	sys->bg_target.lefttop = vec2(min.x - UI_PADDING, min.y - UI_PADDING);
+	sys->bg_target.size    = vec2(max.x - min.x + 2 * UI_PADDING, max.y - min.y + 2 * UI_PADDING);
+}
+
+void ui_Advance(System_UI *sys, Duration deltaTime) {
+	Vec2 lt = sys->bg.lefttop, rb = vec2_Add(sys->bg.lefttop, sys->bg.size);
+	Vec2 lt_target = sys->bg_target.lefttop, rb_target = vec2_Add(sys->bg_target.lefttop, sys->bg_target.size);
+	lt.x = fadeto(lt.x, lt_target.x, deltaTime);
+	lt.y = fadeto(lt.y, lt_target.y, deltaTime);
+	rb.x = fadeto(rb.x, rb_target.x, deltaTime);
+	rb.y = fadeto(rb.y, rb_target.y, deltaTime);
+
+	sys->bg.lefttop = lt;
+	sys->bg.size    = vec2_Minus(rb, lt);
+
+	for (int i = 0; i < ui_StateCount; i++)
+		for (int j = 0; j < vector_Size(sys->parts[i]); j++) {
+			ui_Part *part = (ui_Part *)vector_At(sys->parts[i], j);
+			// Hover check
+			if (box2_Contains(part->box, input_MousePosition(sys->super->input))) {
+				part->hovered  = true;
+				part->progress = dmin(part->progress + UI_BUTTON_HOVER_SPEED * duration_Seconds(deltaTime), 1.0);
+			} else {
+				part->hovered  = false;
+				part->progress = dmax(part->progress - UI_BUTTON_HOVER_SPEED * duration_Seconds(deltaTime), 0.0);
+			}
+
+			part->update(sys, part, part->user, deltaTime);
+		}
+}
+
+
+void ui_PushState(System_UI *sys, ui_State push_state) {
+	vector_Push(sys->state, &push_state);
+	_ui_UpdateBgTarget(sys);
+}
+void ui_PopState(System_UI *sys) {
+	if (vector_Size(sys->state) == 0)
+		WARN("state stack empty");
+	else {
+		vector_Pop(sys->state, NULL);
+		_ui_UpdateBgTarget(sys);
+	}
+}
+
+ui_State ui_CurrentState(System_UI *sys) {
+	if (vector_Size(sys->state) == 0)
+		WARN("state stack empty");
+	return *(ui_State *)vector_Back(sys->state);
+}
diff --git a/ui.h b/ui.h
index 537e21a..be6b761 100644
--- a/ui.h
+++ b/ui.h
@@ -15,26 +15,86 @@ typedef enum {
 	ui_Running,
 	ui_Paused,
 	ui_TitleMenu,
+	ui_SceneSelect,
+	ui_Options,
 	ui_StateCount // Keep at last
-} ui_States;
+} ui_State;
+
+extern const char *ui_StateTitle[ui_StateCount];
 
 
 // Forward declarations
-typedef struct _ui_Button ui_Button;
 typedef struct _System_UI System_UI;
 
 // a Part of the UI.
-typedef struct {
-	void *user;                               // user data
-	void (*draw)(System_UI *sys, void *user); // called when the UI is to be drawn
+typedef struct _ui_Part {
+	Box2      box;                                                                             // bounding box of the UI element
+	float     progress;                                                                        // hover process, from 0 to 1
+	bool      hovered;                                                                         // Whether the part is hovered
+	uintptr_t user;                                                                            // user data
+	void (*draw)(System_UI *sys, struct _ui_Part *part, uintptr_t user);                       // called when the UI is to be drawn
+	void (*update)(System_UI *sys, struct _ui_Part *part, uintptr_t user, Duration deltatime); // called when the element is to be updated
+	void (*free)(System_UI *sys, struct _ui_Part *part, uintptr_t user);                       // called when the element is to be deleted
 } ui_Part;
 
 // UI system with its own state machine.
 typedef struct _System_UI {
 	App           *super;
-	vector_Vector *parts[ui_StateCount];
+	vector_Vector *parts[ui_StateCount]; // A vector of ui_Parts
+	vector_Vector *state;                // A stack of ui_States
+
+	Box2 bg, bg_target;
 } System_UI;
 
+System_UI *ui_NewSystem(App *super);
+void       ui_DeleteSystem(System_UI *sys);
+
+// Clears and refills every menu with its widgets
+void ui_RebuildUI(System_UI *sys);
+
+void ui_PushPart(System_UI *sys, ui_State layer, ui_Part part);
+
+void ui_Advance(System_UI *sys, Duration deltaTime);
+void ui_Render(System_UI *sys);
+
+void ui_PushState(System_UI *sys, ui_State push_state);
+void ui_PopState(System_UI *sys);
+
+ui_State ui_CurrentState(System_UI *sys);
+
+
+#define UI_PADDING    (20)
+#define UI_BACKGROUND (0)
+
+#define UI_BACKGROUND_FADE_MOVE_SPEED (20.0)
+#define UI_BUTTON_HOVER_SPEED         (5.0)
+
+#define UI_BUTTON_BACKGROUND (RGB(50, 50, 50))
+#define UI_BUTTON_HOVERED    (RGB(75, 75, 75))
+#define UI_BUTTON_CLICKED    (RGB(90, 90, 90))
+
+typedef void (*ui_ActionCallback)(System_UI *sys, ui_Part *part, uintptr_t data);
+
+// An Action to pop the UI state stack once
+void ui_Action_PopState(System_UI *sys, ui_Part *part, uintptr_t data);
+// An Action to push the state, casted directly from data
+void ui_Action_PushState(System_UI *sys, ui_Part *part, uintptr_t data);
+
+
+// Allocated & passed in as user data for ui part
+typedef struct _ui_Button {
+	ui_ActionCallback callback;
+	uintptr_t         callback_data;
+
+	uint32_t bg, bg_hover, clicked;
+	char    *label; // Allocated & copied
+	bool     left_aligned;
+} ui_Button;
+
+ui_Part ui_Button_New(Box2 box, const char *label, ui_ActionCallback callback, uintptr_t data);
+ui_Part ui_Button_NewLeftAligned(Box2 box, const char *label, ui_ActionCallback callback, uintptr_t data);
+// ui_Part ui_Button_NewColored(Box2 box, const char *label, uint32_t bg, uint32_t hover, uint32_t clicked, ui_ActionCallback callback, uintptr_t data);
+
 
 #ifdef __cplusplus
 }
diff --git a/ui_action.c b/ui_action.c
new file mode 100644
index 0000000..26f5b41
--- /dev/null
+++ b/ui_action.c
@@ -0,0 +1,10 @@
+
+#include "ui.h"
+
+
+void ui_Action_PopState(System_UI *sys, ui_Part *part, uintptr_t data) {
+	ui_PopState(sys);
+}
+void ui_Action_PushState(System_UI *sys, ui_Part *part, uintptr_t data) {
+	ui_PushState(sys, (ui_State)data);
+}
diff --git a/ui_build.c b/ui_build.c
new file mode 100644
index 0000000..62574fd
--- /dev/null
+++ b/ui_build.c
@@ -0,0 +1,29 @@
+
+#include "ui.h"
+#include "app.h"
+#include "util/vector.h"
+
+
+static void _ui_Action_ResumeGame(System_UI *sys, ui_Part *part, uintptr_t data) {
+	ui_Action_PopState(sys, part, data);
+	sys->super->paused = false;
+}
+
+
+#define PUSH_UI(part) \
+	_p0 = (part);     \
+	vector_Push(p, &_p0);
+
+void ui_RebuildUI(System_UI *sys) {
+	vector_Vector *p;
+	ui_Part        _p0;
+
+
+	// Paused
+	p = sys->parts[ui_Paused];
+	vector_Clear(p);
+	PUSH_UI(ui_Button_New(
+		box2_FromCenter(vec2(SCREEN_WIDTH / 2.0, 300), vec2(400, 50)),
+		"Resume Game",
+		&_ui_Action_ResumeGame, 0))
+}
diff --git a/ui_button.c b/ui_button.c
new file mode 100644
index 0000000..63153c5
--- /dev/null
+++ b/ui_button.c
@@ -0,0 +1,45 @@
+
+#include "input.h"
+#include "types.h"
+#include "ui.h"
+#include "app.h"
+
+
+#ifndef RGB
+#define RGB(r, g, b) ((r) | ((g) << 8) | ((b) << 16))
+#endif
+
+
+void _ui_Button_Draw(System_UI *sys, ui_Part *part, uintptr_t user); // Defined in ui_render.cpp
+void _ui_Button_Update(System_UI *sys, ui_Part *part, uintptr_t user, Duration deltatime) {
+	ui_Button *b = (ui_Button *)user;
+	if (part->hovered && sys->super->input->keys[input_Key_LeftMouse] == JustReleased)
+		b->callback(sys, part, b->callback_data);
+}
+void _ui_Button_Free(System_UI *sys, ui_Part *part, uintptr_t user) {
+	ui_Button *b = (ui_Button *)user;
+	free(b->label);
+	free(b);
+}
+
+
+ui_Part ui_Button_New(Box2 box, const char *label, ui_ActionCallback callback, uintptr_t data) {
+	ui_Button *b     = zero_malloc(sizeof(ui_Button));
+	b->callback      = callback;
+	b->callback_data = data;
+	b->bg            = UI_BUTTON_BACKGROUND;
+	b->bg_hover      = UI_BUTTON_HOVERED;
+	b->clicked       = UI_BUTTON_CLICKED;
+	b->label         = copy_malloc(label);
+	b->left_aligned  = false;
+
+	ui_Part part = {
+		.box      = box,
+		.progress = 0.0f,
+		.user     = (uintptr_t)b,
+		.draw     = &_ui_Button_Draw,
+		.update   = &_ui_Button_Update,
+		.free     = &_ui_Button_Free};
+	return part;
+}
+ui_Part ui_Button_NewLeftAligned(Box2 box, const char *label, ui_ActionCallback callback, uintptr_t data);
diff --git a/ui_render.cpp b/ui_render.cpp
new file mode 100644
index 0000000..38e38c1
--- /dev/null
+++ b/ui_render.cpp
@@ -0,0 +1,75 @@
+
+#include "input.h"
+#include "render_util.h"
+#include "types.h"
+#include "ui.h"
+#include "app.h"
+
+#include <easyx.h>
+
+
+extern "C" void ui_Render(System_UI *sys) {
+	// Background filled rectangle
+	if (sys->bg.size.x > EPS && sys->bg.size.y > EPS) {
+		setfillcolor(UI_BACKGROUND);
+		solidrectangle(
+			(int)round(sys->bg.lefttop.x),
+			(int)round(sys->bg.lefttop.y),
+			(int)round(sys->bg.lefttop.x + sys->bg.size.x),
+			(int)round(sys->bg.lefttop.y + sys->bg.size.y));
+	}
+	if (vector_Size(sys->state) == 0)
+		return;
+
+	ui_State layer = *(ui_State *)vector_Back(sys->state);
+	// Title string
+	setbkcolor(UI_BACKGROUND);
+	render_DrawText(
+		(int)round(sys->bg.lefttop.x),
+		(int)round(sys->bg.lefttop.y) - TEXTHEIGHT,
+		ui_StateTitle[layer]);
+
+	// UI elements
+	// Skip them if the width of background is less than 90% of the target
+	if (sys->bg.size.x > sys->bg_target.size.x * 0.9)
+		for (int i = 0; i < vector_Size(sys->parts[layer]); i++) {
+			ui_Part *part = (ui_Part *)vector_At(sys->parts[layer], i);
+			part->draw(sys, part, part->user);
+		}
+}
+
+extern "C" void _ui_Button_Draw(System_UI *sys, ui_Part *part, void *user) {
+	ui_Button *b = (ui_Button *)user;
+
+	uint32_t color_final = 0;
+
+	if (part->hovered && input_IsPressed(sys->super->input->keys[input_Key_LeftMouse]))
+		// Hovered & pressed
+		color_final = b->clicked;
+	else {
+		// Fade the color
+		int r0 = b->bg & 0xff;
+		int g0 = (b->bg & 0xff00) >> 8;
+		int b0 = (b->bg & 0xff0000) >> 16;
+
+		int r1 = b->bg_hover & 0xff;
+		int g1 = (b->bg_hover & 0xff00) >> 8;
+		int b1 = (b->bg_hover & 0xff0000) >> 16;
+
+		color_final = RGB(
+			(int)((r1 - r0) * sqrt(part->progress) + r0),
+			(int)((g1 - g0) * sqrt(part->progress) + g0),
+			(int)((b1 - b0) * sqrt(part->progress) + b0));
+	}
+
+	setbkcolor(color_final);
+	setfillcolor(color_final);
+	settextcolor(RGB(255, 255, 255));
+
+	solidrectangle(
+		(int)round(part->box.lefttop.x),
+		(int)round(part->box.lefttop.y),
+		(int)round(part->box.lefttop.x + part->box.size.x),
+		(int)round(part->box.lefttop.y + part->box.size.y));
+	render_DrawTextEx(b->label, part->box, (b->left_aligned ? DT_LEFT : DT_CENTER) | DT_VCENTER | DT_SINGLELINE);
+}
diff --git a/util/vector.c b/util/vector.c
index e6db647..4a58989 100644
--- a/util/vector.c
+++ b/util/vector.c
@@ -3,6 +3,7 @@
 
 #include "string.h"
 #include "stdlib.h"
+#include "assert.h"
 
 
 vector_Vector *vector_Create(uintptr_t objectSize) {
@@ -19,6 +20,7 @@ vector_Vector *vector_Create(uintptr_t objectSize) {
 
 // Resizes the underlying buffer to a new capacity
 static inline void __vector_Rebuffer(vector_Vector *vec, uintptr_t newcap) {
+	ASSERT(newcap >= vec->size);
 	void *newbuf = malloc(newcap);
 	memcpy(newbuf, vec->data, vec->size);
 	free(vec->data);
@@ -50,7 +52,7 @@ bool vector_Pop(vector_Vector *vec, void *out_data) {
 
 void vector_Append(vector_Vector *vec, const void *data, uintptr_t n) {
 	uintptr_t oldsize = vec->size, addsize = vec->objectSize * n;
-	vector_Resize(vec, oldsize + addsize);
+	vector_Resize(vec, vector_Size(vec) + n);
 
 	if (data)
 		memcpy(vec->data + oldsize, data, addsize);
@@ -59,7 +61,7 @@ void vector_Append(vector_Vector *vec, const void *data, uintptr_t n) {
 }
 
 void vector_Resize(vector_Vector *vec, uintptr_t size) {
-	uintptr_t newsize = size;
+	uintptr_t newsize = size * vec->objectSize;
 	if (newsize > vec->cap) {
 		// grow the buffer exponentially
 		uint64_t newcap = vec->cap;
@@ -101,3 +103,7 @@ void *vector_At(vector_Vector *vec, uintptr_t i) {
 void *vector_Data(vector_Vector *vec) {
 	return vec->data;
 }
+
+void *vector_Back(vector_Vector *vec) {
+	return vector_At(vec, vector_Size(vec) - 1);
+}
diff --git a/util/vector.h b/util/vector.h
index d59d61f..31fbd05 100644
--- a/util/vector.h
+++ b/util/vector.h
@@ -68,6 +68,11 @@ void *vector_At(vector_Vector *vec, uintptr_t i);
 // Data returns the data buffer.
 void *vector_Data(vector_Vector *vec);
 
+// Back returns the last element in the vector.
+//
+// No boundary test is performed.
+void *vector_Back(vector_Vector *vec);
+
 
 #ifdef __cplusplus
 }