diff --git a/src/main.rs b/src/main.rs
index 6547cb8..bb9b865 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,6 +1,3 @@
-// Zoom lands in pieces; Task 3 removes this.
-#![allow(dead_code)]
-
 mod capture;
 mod config;
 mod detach;
@@ -66,6 +63,11 @@ enum Command {
         #[command(flatten)]
         common: Common,
     },
+    /// Freeze the screen and zoom in: wheel, drag, f, c, 0, Esc.
+    Zoom {
+        #[command(flatten)]
+        common: Common,
+    },
     /// Report what the compositor and system support.
     Doctor,
     /// Internal: the process that shows preview thumbnails.
@@ -106,6 +108,7 @@ enum Mode {
     Screen { all: bool },
     Region,
     Window,
+    Zoom,
 }
 
 fn main() -> ExitCode {
@@ -139,6 +142,7 @@ fn run(cli: Cli) -> Result<()> {
         Command::Screen { all, common } => capture(Mode::Screen { all }, common),
         Command::Region { common } => capture(Mode::Region, common),
         Command::Window { common } => capture(Mode::Window, common),
+        Command::Zoom { common } => capture(Mode::Zoom, common),
     }
 }
 
@@ -188,6 +192,20 @@ fn capture(mode: Mode, common: Common) -> Result<()> {
             }
         }
         Mode::Window => window_shot(&outputs, cursor)?,
+        Mode::Zoom => {
+            let focused = focused_output(&outputs);
+            let frame = wl.capture(&outputs[focused..=focused], cursor)?.remove(0);
+            match zoom::run(&mut wl, &outputs[focused], &frame, &config.zoom)? {
+                zoom::Outcome::Capture(r) => {
+                    let source = frame.output.name.clone();
+                    (vec![(frame.to_rgba(r), None)], 0, source)
+                }
+                zoom::Outcome::Leave => {
+                    tracing::info!("left zoom without capturing");
+                    return Ok(());
+                }
+            }
+        }
     };
     drop(wl);
 
@@ -313,6 +331,8 @@ mod tests {
         assert!(parses(&["screen", "--all", "--no-preview"]));
         assert!(parses(&["window", "--cursor", "--delay", "2"]));
         assert!(!parses(&["window", "--clipboard-only", "-o", "a.png"]));
+        assert!(parses(&["zoom", "--cursor", "--no-preview"]));
+        assert!(!parses(&["zoom", "--clipboard-only", "-o", "a.png"]));
     }
 
     #[test]
diff --git a/src/wayland.rs b/src/wayland.rs
index a5b986b..812b874 100644
--- a/src/wayland.rs
+++ b/src/wayland.rs
@@ -42,6 +42,7 @@ use crate::error::HintExt;
 use crate::frame::OutputGeom;
 use crate::host::Host;
 use crate::region::Overlay;
+use crate::zoom::Zoom;
 
 /// An output as valw sees it.
 #[derive(Debug, Clone)]
@@ -80,6 +81,8 @@ pub struct State {
     pub captures: Vec<Pending>,
     /// The region selection overlay, while it is open.
     pub overlay: Option<Overlay>,
+    /// Zoom mode, while it is open.
+    pub zoom: Option<Zoom>,
     /// The preview thumbnails, in the preview host process.
     pub preview: Option<Host>,
 }
@@ -111,6 +114,7 @@ impl Wayland {
             pointer: None,
             captures: Vec::new(),
             overlay: None,
+            zoom: None,
             preview: None,
         };
         // Two round trips: one for wl_output, one for the xdg-output details.
@@ -297,6 +301,9 @@ impl CompositorHandler for State {
         if let Some(overlay) = &mut self.overlay {
             overlay.frame_done(surface, qh);
         }
+        if let Some(zoom) = &mut self.zoom {
+            zoom.frame_done(surface, qh);
+        }
         if let Some(preview) = &mut self.preview {
             preview.frame_done(surface, qh);
         }
@@ -338,6 +345,9 @@ impl LayerShellHandler for State {
         if let Some(overlay) = &mut self.overlay {
             overlay.cancel();
         }
+        if let Some(zoom) = &mut self.zoom {
+            zoom.closed(layer);
+        }
         if let Some(preview) = &mut self.preview {
             preview.closed(layer);
         }
@@ -354,6 +364,9 @@ impl LayerShellHandler for State {
         if let Some(overlay) = &mut self.overlay {
             overlay.configure(layer, configure.new_size, qh);
         }
+        if let Some(zoom) = &mut self.zoom {
+            zoom.configure(layer, configure.new_size, qh);
+        }
         if let Some(preview) = &mut self.preview {
             preview.configure(layer, qh);
         }
@@ -444,18 +457,20 @@ impl KeyboardHandler for State {
     fn press_key(
         &mut self,
         _: &Connection,
-        _: &QueueHandle<Self>,
+        qh: &QueueHandle<Self>,
         _: &wl_keyboard::WlKeyboard,
         _: u32,
         event: KeyEvent,
     ) {
-        let Some(overlay) = &mut self.overlay else {
-            return;
-        };
-        match event.keysym {
-            Keysym::Escape => overlay.cancel(),
-            Keysym::space => overlay.switch_to_window(),
-            _ => {}
+        if let Some(overlay) = &mut self.overlay {
+            match event.keysym {
+                Keysym::Escape => overlay.cancel(),
+                Keysym::space => overlay.switch_to_window(),
+                _ => {}
+            }
+        }
+        if let Some(zoom) = &mut self.zoom {
+            zoom.key(event.keysym, qh);
         }
     }
 
@@ -485,10 +500,13 @@ impl KeyboardHandler for State {
         _: &QueueHandle<Self>,
         _: &wl_keyboard::WlKeyboard,
         _: u32,
-        _: Modifiers,
+        modifiers: Modifiers,
         _: RawModifiers,
         _: u32,
     ) {
+        if let Some(zoom) = &mut self.zoom {
+            zoom.modifiers(modifiers);
+        }
     }
 }
 
@@ -503,6 +521,9 @@ impl PointerHandler for State {
         if let Some(overlay) = &mut self.overlay {
             overlay.pointer(events, self.cursor_device.as_ref(), qh);
         }
+        if let Some(zoom) = &mut self.zoom {
+            zoom.pointer(events, self.cursor_device.as_ref(), qh);
+        }
         if self.preview.is_some() {
             crate::host::pointer(self, events, qh);
         }
diff --git a/src/zoom/mod.rs b/src/zoom/mod.rs
index 429c905..07f683e 100644
--- a/src/zoom/mod.rs
+++ b/src/zoom/mod.rs
@@ -2,3 +2,335 @@
 
 pub mod gl;
 pub mod view;
+
+use std::sync::Arc;
+use std::time::Instant;
+
+use anyhow::{Context, Result};
+use smithay_client_toolkit::{
+    compositor::FrameCallbackData,
+    seat::{
+        keyboard::{Keysym, Modifiers},
+        pointer::{BTN_LEFT, PointerEvent, PointerEventKind},
+    },
+    shell::{
+        WaylandSurface,
+        wlr_layer::{Anchor, KeyboardInteractivity, Layer, LayerSurface},
+    },
+};
+use wayland_client::{QueueHandle, protocol::wl_surface::WlSurface};
+use wayland_protocols::wp::cursor_shape::v1::client::wp_cursor_shape_device_v1::{
+    Shape, WpCursorShapeDeviceV1,
+};
+use wayland_protocols::wp::viewporter::client::wp_viewport::WpViewport;
+
+use self::view::{Animated, View};
+use crate::config;
+use crate::error::HintExt;
+use crate::frame::{Frame, PixelRect};
+use crate::wayland::{Output, State, Wayland};
+
+pub const NAMESPACE: &str = "valw-zoom";
+
+/// How a zoom session ended.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Outcome {
+    /// `c`: this part of the frame.
+    Capture(PixelRect),
+    /// Esc or `q`: nothing to save.
+    Leave,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Command {
+    Leave,
+    Capture,
+    Flashlight,
+    Reset,
+}
+
+pub fn command(key: Keysym) -> Option<Command> {
+    match key {
+        Keysym::Escape | Keysym::q => Some(Command::Leave),
+        Keysym::c => Some(Command::Capture),
+        Keysym::f => Some(Command::Flashlight),
+        Keysym::_0 => Some(Command::Reset),
+        _ => None,
+    }
+}
+
+pub struct Zoom {
+    // Dropped first: the EGL surface goes before the wl_surface under it.
+    renderer: gl::Renderer,
+    layer: LayerSurface,
+    viewport: WpViewport,
+    /// Frame size in physical px.
+    size: (u32, u32),
+    /// Buffer px per surface-local (logical) px.
+    ratio: f64,
+    view: Animated,
+    step: f64,
+    /// Pointer position in buffer px.
+    pointer: (f64, f64),
+    /// Last pointer position while the left button pans.
+    panning: Option<(f64, f64)>,
+    flashlight: bool,
+    /// Flashlight radius in logical px.
+    radius: f64,
+    ctrl: bool,
+    configured: bool,
+    frame_pending: bool,
+    dirty: bool,
+    last_draw: Option<Instant>,
+    outcome: Option<Outcome>,
+    error: Option<anyhow::Error>,
+}
+
+/// Shows `frame` (the frozen `output`) until the user leaves or captures.
+pub fn run(
+    wl: &mut Wayland,
+    output: &Output,
+    frame: &Frame,
+    config: &config::Zoom,
+) -> Result<Outcome> {
+    let qh = wl.queue.handle();
+    let s = &wl.state;
+    let layer_shell = s
+        .layer_shell
+        .as_ref()
+        .context("compositor does not support wlr-layer-shell")
+        .hint("run `valw doctor` to see what the compositor supports")?;
+    let viewporter = s
+        .viewporter
+        .as_ref()
+        .context("compositor does not support wp-viewporter")
+        .hint("run `valw doctor` to see what the compositor supports")?;
+    let surface = s.compositor.create_surface(&qh);
+    let layer = layer_shell.create_layer_surface(
+        &qh,
+        surface,
+        Layer::Overlay,
+        Some(NAMESPACE),
+        Some(&output.wl),
+    );
+    layer.set_anchor(Anchor::all());
+    layer.set_exclusive_zone(-1);
+    layer.set_keyboard_interactivity(KeyboardInteractivity::Exclusive);
+    layer.commit();
+    let viewport = viewporter.get_viewport(layer.wl_surface(), &qh, ());
+    let pixels: &Arc<_> = &frame.pixels;
+    let size = pixels.dimensions();
+    let renderer = gl::Renderer::new(&wl.conn, layer.wl_surface(), size, pixels)
+        .context("could not start OpenGL ES for zoom")
+        .hint("run `valw doctor` to check EGL")?;
+
+    wl.state.zoom = Some(Zoom {
+        renderer,
+        layer,
+        viewport,
+        size,
+        ratio: 1.0,
+        view: Animated::new(),
+        step: config.scroll_step,
+        pointer: (0.0, 0.0),
+        panning: None,
+        flashlight: false,
+        radius: config.flashlight_radius,
+        ctrl: false,
+        configured: false,
+        frame_pending: false,
+        dirty: true,
+        last_draw: None,
+        outcome: None,
+        error: None,
+    });
+    let result = wl.dispatch_blocking(|s| {
+        s.zoom
+            .as_ref()
+            .is_some_and(|z| z.outcome.is_some() || z.error.is_some())
+    });
+    let ended = wl.state.zoom.take().map(|z| (z.outcome, z.error));
+    // Dropping the zoom destroys its surface; make that visible right away.
+    let _ = wl.conn.flush();
+    result?;
+    match ended {
+        Some((_, Some(e))) => Err(e),
+        Some((Some(outcome), None)) => Ok(outcome),
+        _ => Ok(Outcome::Leave),
+    }
+}
+
+impl Zoom {
+    fn is(&self, surface: &WlSurface) -> bool {
+        self.layer.wl_surface() == surface
+    }
+
+    fn sizef(&self) -> (f64, f64) {
+        (self.size.0 as f64, self.size.1 as f64)
+    }
+
+    pub fn configure(&mut self, layer: &LayerSurface, (w, h): (u32, u32), qh: &QueueHandle<State>) {
+        if !self.is(layer.wl_surface()) {
+            return;
+        }
+        if w > 0 {
+            self.ratio = self.size.0 as f64 / w as f64;
+        }
+        // Show the physical-size buffer 1:1 on the logical-size surface.
+        self.viewport.set_destination(w as i32, h as i32);
+        self.configured = true;
+        self.request_draw(qh);
+    }
+
+    pub fn closed(&mut self, layer: &LayerSurface) {
+        if self.is(layer.wl_surface()) {
+            self.outcome.get_or_insert(Outcome::Leave);
+        }
+    }
+
+    pub fn frame_done(&mut self, surface: &WlSurface, qh: &QueueHandle<State>) {
+        if !self.is(surface) {
+            return;
+        }
+        self.frame_pending = false;
+        if self.dirty {
+            self.draw(qh);
+        }
+    }
+
+    fn request_draw(&mut self, qh: &QueueHandle<State>) {
+        self.dirty = true;
+        if self.configured && !self.frame_pending {
+            self.draw(qh);
+        }
+    }
+
+    fn draw(&mut self, qh: &QueueHandle<State>) {
+        let now = Instant::now();
+        let dt = self
+            .last_draw
+            .map_or(0.0, |t| now.duration_since(t).as_secs_f64().min(0.1));
+        let moving = self.view.step(dt);
+        self.last_draw = moving.then_some(now);
+        let surface = self.layer.wl_surface();
+        surface.frame(qh, FrameCallbackData(surface.clone()));
+        let flashlight =
+            self.flashlight
+                .then_some((self.pointer.0, self.pointer.1, self.radius * self.ratio));
+        if let Err(e) = self.renderer.draw(&self.view.shown, flashlight) {
+            self.error = Some(e);
+            return;
+        }
+        self.frame_pending = true;
+        self.dirty = moving;
+    }
+
+    pub fn pointer(
+        &mut self,
+        events: &[PointerEvent],
+        cursor: Option<&WpCursorShapeDeviceV1>,
+        qh: &QueueHandle<State>,
+    ) {
+        for event in events {
+            if !self.is(&event.surface) {
+                continue;
+            }
+            let p = (event.position.0 * self.ratio, event.position.1 * self.ratio);
+            match event.kind {
+                PointerEventKind::Enter { serial } => {
+                    if let Some(cursor) = cursor {
+                        cursor.set_shape(serial, Shape::Default);
+                    }
+                    self.pointer = p;
+                }
+                PointerEventKind::Press {
+                    button: BTN_LEFT,
+                    serial,
+                    ..
+                } => {
+                    self.panning = Some(p);
+                    if let Some(cursor) = cursor {
+                        cursor.set_shape(serial, Shape::Grabbing);
+                    }
+                }
+                PointerEventKind::Release {
+                    button: BTN_LEFT,
+                    serial,
+                    ..
+                } => {
+                    self.panning = None;
+                    if let Some(cursor) = cursor {
+                        cursor.set_shape(serial, Shape::Default);
+                    }
+                }
+                PointerEventKind::Motion { .. } => {
+                    self.pointer = p;
+                    if let Some(last) = self.panning {
+                        let size = self.sizef();
+                        let moved = self.view.target.pan((p.0 - last.0, p.1 - last.1), size);
+                        self.view.set(moved);
+                        self.panning = Some(p);
+                        self.request_draw(qh);
+                    } else if self.flashlight {
+                        self.request_draw(qh);
+                    }
+                }
+                PointerEventKind::Axis { vertical, .. } => {
+                    let steps = view::scroll_steps(vertical.value120, vertical.absolute);
+                    if steps == 0.0 {
+                        continue;
+                    }
+                    if self.ctrl {
+                        self.radius = view::radius(self.radius, steps, self.step);
+                    } else {
+                        let size = self.sizef();
+                        self.view.target = self.view.target.zoom_at(p, self.step.powf(steps), size);
+                    }
+                    self.request_draw(qh);
+                }
+                _ => {}
+            }
+        }
+    }
+
+    pub fn key(&mut self, key: Keysym, qh: &QueueHandle<State>) {
+        match command(key) {
+            Some(Command::Leave) => {
+                self.outcome.get_or_insert(Outcome::Leave);
+            }
+            Some(Command::Capture) => {
+                let rect = self.view.target.visible_rect(self.size);
+                tracing::info!("zoom capture {rect:?} at {:.2}x", self.view.target.scale);
+                self.outcome.get_or_insert(Outcome::Capture(rect));
+            }
+            Some(Command::Flashlight) => {
+                self.flashlight = !self.flashlight;
+                self.request_draw(qh);
+            }
+            Some(Command::Reset) => {
+                self.view.target = View::IDENTITY;
+                self.request_draw(qh);
+            }
+            None => {}
+        }
+    }
+
+    pub fn modifiers(&mut self, modifiers: Modifiers) {
+        self.ctrl = modifiers.ctrl;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn keys() {
+        assert_eq!(command(Keysym::Escape), Some(Command::Leave));
+        assert_eq!(command(Keysym::q), Some(Command::Leave));
+        assert_eq!(command(Keysym::c), Some(Command::Capture));
+        assert_eq!(command(Keysym::f), Some(Command::Flashlight));
+        assert_eq!(command(Keysym::_0), Some(Command::Reset));
+        assert_eq!(command(Keysym::a), None);
+    }
+}