diff --git a/include/ghostty.h b/include/ghostty.h
index 43d478ab1..2cb915ddd 100644
--- a/include/ghostty.h
+++ b/include/ghostty.h
@@ -276,6 +276,7 @@ void *ghostty_app_userdata(ghostty_app_t);
 ghostty_surface_t ghostty_surface_new(ghostty_app_t, ghostty_surface_config_s*);
 void ghostty_surface_free(ghostty_surface_t);
 ghostty_app_t ghostty_surface_app(ghostty_surface_t);
+bool ghostty_surface_transparent(ghostty_surface_t);
 void ghostty_surface_refresh(ghostty_surface_t);
 void ghostty_surface_set_content_scale(ghostty_surface_t, double, double);
 void ghostty_surface_set_focus(ghostty_surface_t, bool);
@@ -290,6 +291,10 @@ void ghostty_surface_request_close(ghostty_surface_t);
 void ghostty_surface_split(ghostty_surface_t, ghostty_split_direction_e);
 void ghostty_surface_split_focus(ghostty_surface_t, ghostty_split_focus_direction_e);
 
+// APIs I'd like to get rid of eventually but are still needed for now.
+// Don't use these unless you know what you're doing.
+void ghostty_set_window_background_blur(ghostty_surface_t, void *);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/macos/Sources/Ghostty/SurfaceView.swift b/macos/Sources/Ghostty/SurfaceView.swift
index 48467e264..25c74b3dc 100644
--- a/macos/Sources/Ghostty/SurfaceView.swift
+++ b/macos/Sources/Ghostty/SurfaceView.swift
@@ -197,6 +197,20 @@ extension Ghostty {
             ghostty_surface_set_size(surface, UInt32(scaledSize.width), UInt32(scaledSize.height))
         }
         
+        override func viewDidMoveToWindow() {
+            guard let window = self.window else { return }
+            guard let surface = self.surface else { return }
+            guard ghostty_surface_transparent(surface) else { return }
+            
+            // Set the window transparency settings
+            window.isOpaque = false
+            window.hasShadow = false
+            window.backgroundColor = .clear
+            
+            // If we have a blur, set the blur
+            ghostty_set_window_background_blur(surface, Unmanaged.passUnretained(window).toOpaque())
+        }
+        
         override func resignFirstResponder() -> Bool {
             let result = super.resignFirstResponder()
             
diff --git a/src/apprt/embedded.zig b/src/apprt/embedded.zig
index f5a8f899a..1b5c3c4b8 100644
--- a/src/apprt/embedded.zig
+++ b/src/apprt/embedded.zig
@@ -470,6 +470,11 @@ pub const CAPI = struct {
         return surface.app;
     }
 
+    /// Returns ture if the surface has transparency set.
+    export fn ghostty_surface_transparent(surface: *Surface) bool {
+        return surface.app.config.@"background-opacity" < 1.0;
+    }
+
     /// Tell the surface that it needs to schedule a render
     export fn ghostty_surface_refresh(surface: *Surface) void {
         surface.refresh();
@@ -565,4 +570,34 @@ pub const CAPI = struct {
     export fn ghostty_surface_split_focus(ptr: *Surface, direction: input.SplitFocusDirection) void {
         ptr.gotoSplit(direction);
     }
+
+    /// Sets the window background blur on macOS to the desired value.
+    /// I do this in Zig as an extern function because I don't know how to
+    /// call these functions in Swift.
+    ///
+    /// This uses an undocumented, non-public API because this is what
+    /// every terminal appears to use, including Terminal.app.
+    export fn ghostty_set_window_background_blur(
+        ptr: *Surface,
+        window: *anyopaque,
+    ) void {
+        const config = ptr.app.config;
+
+        // Do nothing if we don't have background transparency enabled
+        if (config.@"background-opacity" >= 1.0) return;
+
+        // Do nothing if our blur value is zero
+        if (config.@"background-blur-radius" == 0) return;
+
+        const nswindow = objc.Object.fromId(window);
+        _ = CGSSetWindowBackgroundBlurRadius(
+            CGSDefaultConnectionForThread(),
+            nswindow.msgSend(usize, objc.sel("windowNumber"), .{}),
+            @intCast(config.@"background-blur-radius"),
+        );
+    }
+
+    /// See ghostty_set_window_background_blur
+    extern "c" fn CGSSetWindowBackgroundBlurRadius(*anyopaque, usize, c_int) i32;
+    extern "c" fn CGSDefaultConnectionForThread() *anyopaque;
 };
diff --git a/src/apprt/glfw.zig b/src/apprt/glfw.zig
index 47bf9c422..4863e5511 100644
--- a/src/apprt/glfw.zig
+++ b/src/apprt/glfw.zig
@@ -278,7 +278,7 @@ pub const Surface = struct {
             "ghostty",
             null,
             null,
-            Renderer.glfwWindowHints(),
+            Renderer.glfwWindowHints(&app.config),
         ) orelse return glfw.mustGetErrorCode();
         errdefer win.destroy();
 
diff --git a/src/apprt/gtk.zig b/src/apprt/gtk.zig
index 69a9634b4..c3a609c4a 100644
--- a/src/apprt/gtk.zig
+++ b/src/apprt/gtk.zig
@@ -286,6 +286,9 @@ const Window = struct {
     /// The notebook (tab grouping) for this window.
     notebook: *c.GtkNotebook,
 
+    /// The background CSS for the window (if any).
+    css_window_background: ?[]u8 = null,
+
     pub fn init(self: *Window, app: *App) !void {
         // Set up our own state
         self.* = .{
@@ -301,6 +304,27 @@ const Window = struct {
         self.window = gtk_window;
         c.gtk_window_set_title(gtk_window, "Ghostty");
         c.gtk_window_set_default_size(gtk_window, 200, 200);
+
+        // Apply background opacity if we have it
+        if (app.config.@"background-opacity" < 1) {
+            var css = try std.fmt.allocPrint(
+                app.core_app.alloc,
+                ".window-transparent {{ background-color: rgba(0, 0, 0, {d}); }}",
+                .{app.config.@"background-opacity"},
+            );
+            self.css_window_background = css;
+
+            const display = c.gtk_widget_get_display(@ptrCast(window));
+            const provider = c.gtk_css_provider_new();
+            c.gtk_css_provider_load_from_data(provider, css.ptr, @intCast(css.len));
+            c.gtk_style_context_add_provider_for_display(
+                display,
+                @ptrCast(provider),
+                c.GTK_STYLE_PROVIDER_PRIORITY_APPLICATION,
+            );
+            c.gtk_widget_add_css_class(@ptrCast(window), "window-transparent");
+        }
+
         c.gtk_widget_show(window);
         _ = c.g_signal_connect_data(window, "close-request", c.G_CALLBACK(&gtkCloseRequest), self, null, G_CONNECT_DEFAULT);
         _ = c.g_signal_connect_data(window, "destroy", c.G_CALLBACK(&gtkDestroy), self, null, G_CONNECT_DEFAULT);
@@ -326,9 +350,7 @@ const Window = struct {
     }
 
     pub fn deinit(self: *Window) void {
-        // Notify our app we're gone.
-        // TODO
-        _ = self;
+        if (self.css_window_background) |ptr| self.app.core_app.alloc.free(ptr);
     }
 
     /// Add a new tab to this window.
diff --git a/src/cli_args.zig b/src/cli_args.zig
index 70a918369..8afb35e65 100644
--- a/src/cli_args.zig
+++ b/src/cli_args.zig
@@ -144,6 +144,11 @@ fn parseIntoField(
                     0,
                 ),
 
+                f64 => try std.fmt.parseFloat(
+                    f64,
+                    value orelse return error.ValueRequired,
+                ),
+
                 else => unreachable,
             };
 
@@ -298,6 +303,20 @@ test "parseIntoField: unsigned numbers" {
     try testing.expectEqual(@as(u8, 1), data.u8);
 }
 
+test "parseIntoField: floats" {
+    const testing = std.testing;
+    var arena = ArenaAllocator.init(testing.allocator);
+    defer arena.deinit();
+    const alloc = arena.allocator();
+
+    var data: struct {
+        f64: f64,
+    } = undefined;
+
+    try parseIntoField(@TypeOf(data), alloc, &data, "f64", "1");
+    try testing.expectEqual(@as(f64, 1.0), data.f64);
+}
+
 test "parseIntoField: optional field" {
     const testing = std.testing;
     var arena = ArenaAllocator.init(testing.allocator);
diff --git a/src/config.zig b/src/config.zig
index c1c9f1f72..cd92cbbf4 100644
--- a/src/config.zig
+++ b/src/config.zig
@@ -63,6 +63,24 @@ pub const Config = struct {
     /// The color of the cursor. If this is not set, a default will be chosen.
     @"cursor-color": ?Color = null,
 
+    /// The opacity level (opposite of transparency) of the background.
+    /// A value of 1 is fully opaque and a value of 0 is fully transparent.
+    /// A value less than 0 or greater than 1 will be clamped to the nearest
+    /// valid value.
+    ///
+    /// Changing this value at runtime (and reloading config) will only
+    /// affect new windows, tabs, and splits.
+    @"background-opacity": f64 = 1.0,
+
+    /// A positive value enables blurring of the background when
+    /// background-opacity is less than 1. The value is the blur radius to
+    /// apply. A value of 20 is reasonable for a good looking blur.
+    /// Higher values will cause strange rendering issues as well as
+    /// performance issues.
+    ///
+    /// This is only supported on macOS.
+    @"background-blur-radius": u8 = 0,
+
     /// The command to run, usually a shell. If this is not an absolute path,
     /// it'll be looked up in the PATH. If this is not set, a default will
     /// be looked up from your system. The rules for the default lookup are:
@@ -754,6 +772,7 @@ pub const Config = struct {
         switch (@typeInfo(T)) {
             inline .Bool,
             .Int,
+            .Float,
             => return src,
 
             .Optional => |info| return try cloneValue(
@@ -879,6 +898,7 @@ fn equal(comptime T: type, old: T, new: T) bool {
 
         inline .Bool,
         .Int,
+        .Float,
         .Enum,
         => return old == new,
 
diff --git a/src/renderer/Metal.zig b/src/renderer/Metal.zig
index 7d75219bf..89a200be0 100644
--- a/src/renderer/Metal.zig
+++ b/src/renderer/Metal.zig
@@ -132,6 +132,7 @@ pub const DerivedConfig = struct {
     font_thicken: bool,
     cursor_color: ?terminal.color.RGB,
     background: terminal.color.RGB,
+    background_opacity: f64,
     foreground: terminal.color.RGB,
     selection_background: ?terminal.color.RGB,
     selection_foreground: ?terminal.color.RGB,
@@ -143,6 +144,7 @@ pub const DerivedConfig = struct {
         _ = alloc_gpa;
 
         return .{
+            .background_opacity = @max(0, @min(1, config.@"background-opacity")),
             .font_thicken = config.@"font-thicken",
 
             .cursor_color = if (config.@"cursor-color") |col|
@@ -171,11 +173,10 @@ pub const DerivedConfig = struct {
 };
 
 /// Returns the hints that we want for this
-pub fn glfwWindowHints() glfw.Window.Hints {
+pub fn glfwWindowHints(config: *const configpkg.Config) glfw.Window.Hints {
     return .{
         .client_api = .no_api,
-        // .cocoa_graphics_switching = builtin.os.tag == .macos,
-        // .cocoa_retina_framebuffer = true,
+        .transparent_framebuffer = config.@"background-opacity" < 1,
     };
 }
 
@@ -196,7 +197,7 @@ pub fn init(alloc: Allocator, options: renderer.Options) !Metal {
         const CAMetalLayer = objc.Class.getClass("CAMetalLayer").?;
         const swapchain = CAMetalLayer.msgSend(objc.Object, objc.sel("layer"), .{});
         swapchain.setProperty("device", device.value);
-        swapchain.setProperty("opaque", true);
+        swapchain.setProperty("opaque", options.config.background_opacity >= 1);
 
         // disable v-sync
         swapchain.setProperty("displaySyncEnabled", false);
@@ -628,7 +629,7 @@ pub fn render(
                     .red = @as(f32, @floatFromInt(critical.bg.r)) / 255,
                     .green = @as(f32, @floatFromInt(critical.bg.g)) / 255,
                     .blue = @as(f32, @floatFromInt(critical.bg.b)) / 255,
-                    .alpha = 1.0,
+                    .alpha = self.config.background_opacity,
                 });
             }
 
@@ -943,30 +944,25 @@ pub fn updateCell(
         fg: terminal.color.RGB,
     };
 
+    // True if this cell is selected
+    // TODO(perf): we can check in advance if selection is in
+    // our viewport at all and not run this on every point.
+    const selected: bool = if (selection) |sel| selected: {
+        const screen_point = (terminal.point.Viewport{
+            .x = x,
+            .y = y,
+        }).toScreen(screen);
+
+        break :selected sel.contains(screen_point);
+    } else false;
+
     // The colors for the cell.
     const colors: BgFg = colors: {
-        // If we have a selection, then we need to check if this
-        // cell is selected.
-        // TODO(perf): we can check in advance if selection is in
-        // our viewport at all and not run this on every point.
-        var selection_res: ?BgFg = sel_colors: {
-            if (selection) |sel| {
-                const screen_point = (terminal.point.Viewport{
-                    .x = x,
-                    .y = y,
-                }).toScreen(screen);
-
-                // If we are selected, we our colors are just inverted fg/bg
-                if (sel.contains(screen_point)) {
-                    break :sel_colors BgFg{
-                        .bg = self.config.selection_background orelse self.config.foreground,
-                        .fg = self.config.selection_foreground orelse self.config.background,
-                    };
-                }
-            }
-
-            break :sel_colors null;
-        };
+        // If we are selected, we our colors are just inverted fg/bg
+        var selection_res: ?BgFg = if (selected) .{
+            .bg = self.config.selection_background orelse self.config.foreground,
+            .fg = self.config.selection_foreground orelse self.config.background,
+        } else null;
 
         const res: BgFg = selection_res orelse if (!cell.attrs.inverse) .{
             // In normal mode, background and fg match the cell. We
@@ -998,11 +994,37 @@ pub fn updateCell(
 
     // If the cell has a background, we always draw it.
     if (colors.bg) |rgb| {
+        // Determine our background alpha. If we have transparency configured
+        // then this is dynamic depending on some situations. This is all
+        // in an attempt to make transparency look the best for various
+        // situations. See inline comments.
+        const bg_alpha: u8 = bg_alpha: {
+            if (self.config.background_opacity >= 1) break :bg_alpha alpha;
+
+            // If we're selected, we do not apply background opacity
+            if (selected) break :bg_alpha alpha;
+
+            // If we're reversed, do not apply background opacity
+            if (cell.attrs.inverse) break :bg_alpha alpha;
+
+            // If we have a background and its not the default background
+            // then we apply background opacity
+            if (cell.attrs.has_bg and !std.meta.eql(rgb, self.config.background)) {
+                break :bg_alpha alpha;
+            }
+
+            // We apply background opacity.
+            var bg_alpha: f64 = @floatFromInt(alpha);
+            bg_alpha *= self.config.background_opacity;
+            bg_alpha = @ceil(bg_alpha);
+            break :bg_alpha @intFromFloat(bg_alpha);
+        };
+
         self.cells_bg.appendAssumeCapacity(.{
             .mode = .bg,
             .grid_pos = .{ @as(f32, @floatFromInt(x)), @as(f32, @floatFromInt(y)) },
             .cell_width = cell.widthLegacy(),
-            .color = .{ rgb.r, rgb.g, rgb.b, alpha },
+            .color = .{ rgb.r, rgb.g, rgb.b, bg_alpha },
         });
     }
 
diff --git a/src/renderer/OpenGL.zig b/src/renderer/OpenGL.zig
index 84c7ad76e..3d06901de 100644
--- a/src/renderer/OpenGL.zig
+++ b/src/renderer/OpenGL.zig
@@ -238,6 +238,7 @@ pub const DerivedConfig = struct {
     font_thicken: bool,
     cursor_color: ?terminal.color.RGB,
     background: terminal.color.RGB,
+    background_opacity: f64,
     foreground: terminal.color.RGB,
     selection_background: ?terminal.color.RGB,
     selection_foreground: ?terminal.color.RGB,
@@ -249,6 +250,7 @@ pub const DerivedConfig = struct {
         _ = alloc_gpa;
 
         return .{
+            .background_opacity = @max(0, @min(1, config.@"background-opacity")),
             .font_thicken = config.@"font-thicken",
 
             .cursor_color = if (config.@"cursor-color") |col|
@@ -461,7 +463,7 @@ fn resetCellsLRU(self: *OpenGL) void {
 }
 
 /// Returns the hints that we want for this
-pub fn glfwWindowHints() glfw.Window.Hints {
+pub fn glfwWindowHints(config: *const configpkg.Config) glfw.Window.Hints {
     return .{
         .context_version_major = 3,
         .context_version_minor = 3,
@@ -469,6 +471,7 @@ pub fn glfwWindowHints() glfw.Window.Hints {
         .opengl_forward_compat = true,
         .cocoa_graphics_switching = builtin.os.tag == .macos,
         .cocoa_retina_framebuffer = true,
+        .transparent_framebuffer = config.@"background-opacity" < 1,
     };
 }
 
@@ -1059,30 +1062,25 @@ pub fn updateCell(
         fg: terminal.color.RGB,
     };
 
+    // True if this cell is selected
+    // TODO(perf): we can check in advance if selection is in
+    // our viewport at all and not run this on every point.
+    const selected: bool = if (selection) |sel| selected: {
+        const screen_point = (terminal.point.Viewport{
+            .x = x,
+            .y = y,
+        }).toScreen(screen);
+
+        break :selected sel.contains(screen_point);
+    } else false;
+
     // The colors for the cell.
     const colors: BgFg = colors: {
-        // If we have a selection, then we need to check if this
-        // cell is selected.
-        // TODO(perf): we can check in advance if selection is in
-        // our viewport at all and not run this on every point.
-        var selection_res: ?BgFg = sel_colors: {
-            if (selection) |sel| {
-                const screen_point = (terminal.point.Viewport{
-                    .x = x,
-                    .y = y,
-                }).toScreen(screen);
-
-                // If we are selected, we our colors are just inverted fg/bg
-                if (sel.contains(screen_point)) {
-                    break :sel_colors BgFg{
-                        .bg = self.config.selection_background orelse self.config.foreground,
-                        .fg = self.config.selection_foreground orelse self.config.background,
-                    };
-                }
-            }
-
-            break :sel_colors null;
-        };
+        // If we are selected, we our colors are just inverted fg/bg
+        var selection_res: ?BgFg = if (selected) .{
+            .bg = self.config.selection_background orelse self.config.foreground,
+            .fg = self.config.selection_foreground orelse self.config.background,
+        } else null;
 
         const res: BgFg = selection_res orelse if (!cell.attrs.inverse) .{
             // In normal mode, background and fg match the cell. We
@@ -1125,10 +1123,34 @@ pub fn updateCell(
 
     // If the cell has a background, we always draw it.
     if (colors.bg) |rgb| {
-        var mode: GPUCellMode = .bg;
+        // Determine our background alpha. If we have transparency configured
+        // then this is dynamic depending on some situations. This is all
+        // in an attempt to make transparency look the best for various
+        // situations. See inline comments.
+        const bg_alpha: u8 = bg_alpha: {
+            if (self.config.background_opacity >= 1) break :bg_alpha alpha;
+
+            // If we're selected, we do not apply background opacity
+            if (selected) break :bg_alpha alpha;
+
+            // If we're reversed, do not apply background opacity
+            if (cell.attrs.inverse) break :bg_alpha alpha;
+
+            // If we have a background and its not the default background
+            // then we apply background opacity
+            if (cell.attrs.has_bg and !std.meta.eql(rgb, self.config.background)) {
+                break :bg_alpha alpha;
+            }
+
+            // We apply background opacity.
+            var bg_alpha: f64 = @floatFromInt(alpha);
+            bg_alpha *= self.config.background_opacity;
+            bg_alpha = @ceil(bg_alpha);
+            break :bg_alpha @intFromFloat(bg_alpha);
+        };
 
         self.cells_bg.appendAssumeCapacity(.{
-            .mode = mode,
+            .mode = .bg,
             .grid_col = @intCast(x),
             .grid_row = @intCast(y),
             .grid_width = cell.widthLegacy(),
@@ -1145,7 +1167,7 @@ pub fn updateCell(
             .bg_r = rgb.r,
             .bg_g = rgb.g,
             .bg_b = rgb.b,
-            .bg_a = alpha,
+            .bg_a = bg_alpha,
         });
     }
 
@@ -1411,7 +1433,7 @@ pub fn draw(self: *OpenGL) !void {
         @as(f32, @floatFromInt(self.draw_background.r)) / 255,
         @as(f32, @floatFromInt(self.draw_background.g)) / 255,
         @as(f32, @floatFromInt(self.draw_background.b)) / 255,
-        1.0,
+        @floatCast(self.config.background_opacity),
     );
     gl.clear(gl.c.GL_COLOR_BUFFER_BIT);