diff --git a/src/log.rs b/src/log.rs
new file mode 100644
index 0000000..18873f3
--- /dev/null
+++ b/src/log.rs
@@ -0,0 +1,155 @@
+use std::fs::{self, File};
+use std::io::IsTerminal;
+use std::path::{Path, PathBuf};
+use std::sync::{Mutex, OnceLock};
+use std::time::{Duration, Instant};
+
+use anyhow::{Context, Result};
+use chrono::{DateTime, Local};
+use tracing::Level;
+use tracing_subscriber::filter::{EnvFilter, filter_fn};
+use tracing_subscriber::prelude::*;
+
+use crate::error::{Hint, render};
+
+/// Number of run logs kept on disk, including the current one.
+const KEEP: usize = 10;
+
+static START: OnceLock<Instant> = OnceLock::new();
+
+/// Time since logging was set up, i.e. roughly since the process started.
+pub fn since_start() -> Duration {
+    START.get_or_init(Instant::now).elapsed()
+}
+
+/// `$XDG_STATE_HOME/valw/logs`, falling back to `~/.local/state/valw/logs`.
+pub fn default_dir() -> PathBuf {
+    std::env::var_os("XDG_STATE_HOME")
+        .map(PathBuf::from)
+        .unwrap_or_else(|| crate::config::home().join(".local/state"))
+        .join("valw/logs")
+}
+
+/// Sets up tracing: this run's log file at `VALW_LOG` level (default info),
+/// plus warnings and errors on stderr. Returns the log file path.
+///
+/// If the log file can't be created, logging falls back to stderr only and
+/// the returned path is `None`; a broken log dir must not block a screenshot.
+pub fn init(dir: &Path, now: DateTime<Local>) -> Option<PathBuf> {
+    START.get_or_init(Instant::now);
+    let file = open_run_log(dir, now);
+
+    // Errors reach stderr through error::render, so only warnings go here.
+    let stderr = tracing_subscriber::fmt::layer()
+        .with_writer(std::io::stderr)
+        .with_ansi(std::io::stderr().is_terminal())
+        .without_time()
+        .with_target(false)
+        .with_filter(filter_fn(|m| *m.level() == Level::WARN));
+
+    let (file_layer, path, failure) = match file {
+        Ok((file, path)) => {
+            let filter =
+                EnvFilter::try_from_env("VALW_LOG").unwrap_or_else(|_| EnvFilter::new("info"));
+            let layer = tracing_subscriber::fmt::layer()
+                .with_writer(Mutex::new(file))
+                .with_ansi(false)
+                .with_filter(filter);
+            (Some(layer), Some(path), None)
+        }
+        Err(e) => (None, None, Some(e)),
+    };
+
+    tracing_subscriber::registry()
+        .with(stderr)
+        .with(file_layer)
+        .init();
+
+    if let Some(e) = failure {
+        tracing::warn!("logging to stderr only: {e:#}");
+    }
+    path
+}
+
+fn open_run_log(dir: &Path, now: DateTime<Local>) -> Result<(File, PathBuf)> {
+    fs::create_dir_all(dir).with_context(|| format!("could not create {}", dir.display()))?;
+    let path = dir.join(log_name(now));
+    let file =
+        File::create(&path).with_context(|| format!("could not create {}", path.display()))?;
+    prune(dir, KEEP)?;
+    Ok((file, path))
+}
+
+/// Log names sort chronologically, so pruning can compare names.
+fn log_name(now: DateTime<Local>) -> String {
+    now.format("%Y-%m-%dT%H-%M-%S%.3f.log").to_string()
+}
+
+/// Deletes all but the `keep` newest `.log` files in `dir`.
+fn prune(dir: &Path, keep: usize) -> Result<()> {
+    let mut logs: Vec<PathBuf> = fs::read_dir(dir)?
+        .filter_map(|e| e.ok().map(|e| e.path()))
+        .filter(|p| p.extension().is_some_and(|x| x == "log"))
+        .collect();
+    logs.sort();
+    let excess = logs.len().saturating_sub(keep);
+    for old in &logs[..excess] {
+        fs::remove_file(old)?;
+    }
+    Ok(())
+}
+
+/// Sends panics to the log and prints them in the usual error format.
+pub fn install_panic_hook(log: Option<PathBuf>) {
+    std::panic::set_hook(Box::new(move |info| {
+        let backtrace = std::backtrace::Backtrace::force_capture();
+        tracing::error!("panic: {info}\n{backtrace}");
+        let err = anyhow::anyhow!("{info}").context(Hint(
+            "this is a bug in valw; please report it with the log".into(),
+        ));
+        eprint!("{}", render(&err, log.as_deref()));
+    }));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use chrono::TimeZone;
+
+    #[test]
+    fn log_names_sort_by_time() {
+        let a = Local.with_ymd_and_hms(2026, 9, 29, 9, 5, 1).unwrap();
+        let b = Local.with_ymd_and_hms(2026, 9, 29, 19, 0, 0).unwrap();
+        assert_eq!(log_name(a), "2026-09-29T09-05-01.000.log");
+        assert!(log_name(a) < log_name(b));
+    }
+
+    #[test]
+    fn prune_keeps_newest() {
+        let dir = tempfile::tempdir().unwrap();
+        for i in 0..12 {
+            File::create(dir.path().join(format!("2026-09-29T10-00-{i:02}.000.log"))).unwrap();
+        }
+        File::create(dir.path().join("notes.txt")).unwrap();
+
+        prune(dir.path(), 10).unwrap();
+
+        let mut left: Vec<String> = fs::read_dir(dir.path())
+            .unwrap()
+            .map(|e| e.unwrap().file_name().into_string().unwrap())
+            .collect();
+        left.sort();
+        assert_eq!(left.len(), 11);
+        assert_eq!(left[0], "2026-09-29T10-00-02.000.log");
+        assert!(left.contains(&"notes.txt".to_string()));
+    }
+
+    #[test]
+    fn open_run_log_fails_on_unwritable_dir() {
+        let dir = tempfile::tempdir().unwrap();
+        let blocker = dir.path().join("file");
+        File::create(&blocker).unwrap();
+        // A regular file where the log dir should be.
+        assert!(open_run_log(&blocker, Local::now()).is_err());
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index 6427c34..d3b6d90 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -3,6 +3,7 @@
 
 mod config;
 mod error;
+mod log;
 
 use clap::Parser;