diff --git a/crates/libtest2-harness/Cargo.toml b/crates/libtest2-harness/Cargo.toml
index 4805b88..889400a 100644
--- a/crates/libtest2-harness/Cargo.toml
+++ b/crates/libtest2-harness/Cargo.toml
@@ -26,6 +26,7 @@ pre-release-replacements = [
 default = []
 json = ["dep:serde", "dep:serde_json"]
 junit = []
+threads = []
 
 [dependencies]
 anstream = "0.3.1"
diff --git a/crates/libtest2-harness/src/case.rs b/crates/libtest2-harness/src/case.rs
index 33cc4a4..48b59fc 100644
--- a/crates/libtest2-harness/src/case.rs
+++ b/crates/libtest2-harness/src/case.rs
@@ -1,6 +1,6 @@
 pub use crate::*;
 
-pub trait Case {
+pub trait Case: Send + Sync + 'static {
     // The name of a test
     //
     // By convention this follows the rules for rust paths; i.e., it should be a series of
diff --git a/crates/libtest2-harness/src/harness.rs b/crates/libtest2-harness/src/harness.rs
index e5d22f2..456fab3 100644
--- a/crates/libtest2-harness/src/harness.rs
+++ b/crates/libtest2-harness/src/harness.rs
@@ -32,7 +32,7 @@ impl Harness {
 
     pub fn main(mut self) -> ! {
         let mut parser = cli::Parser::new(&self.raw);
-        let opts = parse(&mut parser).unwrap_or_else(|err| {
+        let mut opts = parse(&mut parser).unwrap_or_else(|err| {
             eprintln!("{}", err);
             std::process::exit(1)
         });
@@ -52,9 +52,12 @@ impl Harness {
             eprintln!("{}", err);
             std::process::exit(1)
         });
+        if self.cases.len() == 1 {
+            opts.test_threads = Some(std::num::NonZeroUsize::new(1).unwrap());
+        }
 
         if !opts.list {
-            match run(&opts, &self.cases, notifier.as_mut()) {
+            match run(&opts, self.cases, notifier.as_mut()) {
                 Ok(true) => {}
                 Ok(false) => std::process::exit(ERROR_EXIT_CODE),
                 Err(e) => {
@@ -116,7 +119,18 @@ fn parse(parser: &mut cli::Parser) -> cli::Result<libtest_lexarg::TestOpts> {
         }
     }
 
-    test_opts.finish()
+    let mut opts = test_opts.finish()?;
+    // If the platform is single-threaded we're just going to run
+    // the test synchronously, regardless of the concurrency
+    // level.
+    let supports_threads = !cfg!(target_os = "emscripten") && !cfg!(target_family = "wasm");
+    opts.test_threads = if cfg!(feature = "threads") && supports_threads {
+        opts.test_threads
+            .or_else(|| std::thread::available_parallelism().ok())
+    } else {
+        None
+    };
+    Ok(opts)
 }
 
 fn notifier(opts: &libtest_lexarg::TestOpts) -> std::io::Result<Box<dyn notify::Notifier>> {
@@ -200,7 +214,7 @@ fn discover(
 
 fn run(
     opts: &libtest_lexarg::TestOpts,
-    cases: &[Box<dyn Case>],
+    cases: Vec<Box<dyn Case>>,
     notifier: &mut dyn notify::Notifier,
 ) -> std::io::Result<bool> {
     notifier.notify(notify::Event::SuiteStart)?;
@@ -228,6 +242,10 @@ fn run(
         todo!("`--logfile` is not yet supported");
     }
 
+    let threads = opts.test_threads.map(|t| t.get()).unwrap_or(1);
+    let is_multithreaded = 1 < threads;
+    notifier.threaded(is_multithreaded);
+
     let mut state = State::new();
     let run_ignored = match opts.run_ignored {
         libtest_lexarg::RunIgnored::Yes | libtest_lexarg::RunIgnored::Only => true,
@@ -236,28 +254,100 @@ fn run(
     state.run_ignored(run_ignored);
 
     let mut success = true;
-    for case in cases {
-        notifier.notify(notify::Event::CaseStart {
-            name: case.name().to_owned(),
-        })?;
-        let timer = std::time::Instant::now();
+    if is_multithreaded {
+        struct RunningTest {
+            join_handle: std::thread::JoinHandle<()>,
+        }
 
-        let outcome = __rust_begin_short_backtrace(|| case.run(&state));
+        impl RunningTest {
+            fn join(self, event: &mut notify::Event) {
+                if let Err(_) = self.join_handle.join() {
+                    if let notify::Event::CaseComplete {
+                        status, message, ..
+                    } = event
+                    {
+                        if status.is_none() {
+                            *status = Some(notify::RunStatus::Failed);
+                            *message = Some("panicked after reporting success".to_owned());
+                        }
+                    }
+                }
+            }
+        }
 
-        let err = outcome.as_ref().err();
-        let status = err.map(|e| e.status());
-        let message = err.and_then(|e| e.cause().map(|c| c.to_string()));
-        notifier.notify(notify::Event::CaseComplete {
-            name: case.name().to_owned(),
-            mode: notify::CaseMode::Test,
-            status,
-            message,
-            elapsed_s: Some(notify::Elapsed(timer.elapsed())),
-        })?;
+        // Use a deterministic hasher
+        type TestMap = std::collections::HashMap<
+            String,
+            RunningTest,
+            std::hash::BuildHasherDefault<std::collections::hash_map::DefaultHasher>,
+        >;
+
+        let sync_success = std::sync::Arc::new(std::sync::atomic::AtomicBool::new(success));
+        let mut running_tests: TestMap = Default::default();
+        let mut pending = 0;
+        let state = std::sync::Arc::new(state);
+        let (tx, rx) = std::sync::mpsc::channel::<notify::Event>();
+        let mut remaining = std::collections::VecDeque::from(cases);
+        while pending > 0 || !remaining.is_empty() {
+            while pending < threads && !remaining.is_empty() {
+                let case = remaining.pop_front().unwrap();
+                let name = case.name().to_owned();
+
+                let cfg = std::thread::Builder::new().name(name.to_owned());
+                let tx = tx.clone();
+                let case = std::sync::Arc::new(case);
+                let case_fallback = case.clone();
+                let state = state.clone();
+                let state_fallback = state.clone();
+                let sync_success = sync_success.clone();
+                let sync_success_fallback = sync_success.clone();
+                match cfg.spawn(move || {
+                    let mut notifier = SenderNotifier { tx: tx.clone() };
+                    let case_success = run_case(case.as_ref().as_ref(), &state, &mut notifier)
+                        .expect("`SenderNotifier` is infallible");
+                    if !case_success {
+                        sync_success.store(case_success, std::sync::atomic::Ordering::Relaxed);
+                    }
+                }) {
+                    Ok(join_handle) => {
+                        running_tests.insert(name.clone(), RunningTest { join_handle });
+                        pending += 1;
+                    }
+                    Err(e) if e.kind() == std::io::ErrorKind::WouldBlock => {
+                        // `ErrorKind::WouldBlock` means hitting the thread limit on some
+                        // platforms, so run the test synchronously here instead.
+                        let case_success =
+                            run_case(case_fallback.as_ref().as_ref(), &state_fallback, notifier)
+                                .expect("`SenderNotifier` is infallible");
+                        if !case_success {
+                            sync_success_fallback
+                                .store(case_success, std::sync::atomic::Ordering::Relaxed);
+                        }
+                    }
+                    Err(e) => {
+                        return Err(e);
+                    }
+                }
+            }
 
-        success &= status != Some(notify::RunStatus::Failed);
-        if !success && opts.fail_fast {
-            break;
+            let mut event = rx.recv().unwrap();
+            if let notify::Event::CaseComplete { name, .. } = &event {
+                let running_test = running_tests.remove(name).unwrap();
+                running_test.join(&mut event);
+                pending -= 1;
+            }
+            notifier.notify(event)?;
+            success &= sync_success.load(std::sync::atomic::Ordering::SeqCst);
+            if !success && opts.fail_fast {
+                break;
+            }
+        }
+    } else {
+        for case in cases {
+            success &= run_case(case.as_ref(), &state, notifier)?;
+            if !success && opts.fail_fast {
+                break;
+            }
         }
     }
 
@@ -268,6 +358,32 @@ fn run(
     Ok(success)
 }
 
+fn run_case(
+    case: &dyn Case,
+    state: &State,
+    notifier: &mut dyn notify::Notifier,
+) -> std::io::Result<bool> {
+    notifier.notify(notify::Event::CaseStart {
+        name: case.name().to_owned(),
+    })?;
+    let timer = std::time::Instant::now();
+
+    let outcome = __rust_begin_short_backtrace(|| case.run(&state));
+
+    let err = outcome.as_ref().err();
+    let status = err.map(|e| e.status());
+    let message = err.and_then(|e| e.cause().map(|c| c.to_string()));
+    notifier.notify(notify::Event::CaseComplete {
+        name: case.name().to_owned(),
+        mode: notify::CaseMode::Test,
+        status,
+        message,
+        elapsed_s: Some(notify::Elapsed(timer.elapsed())),
+    })?;
+
+    Ok(status != Some(notify::RunStatus::Failed))
+}
+
 /// Fixed frame used to clean the backtrace with `RUST_BACKTRACE=1`.
 #[inline(never)]
 fn __rust_begin_short_backtrace<T, F: FnOnce() -> T>(f: F) -> T {
@@ -276,3 +392,16 @@ fn __rust_begin_short_backtrace<T, F: FnOnce() -> T>(f: F) -> T {
     // prevent this frame from being tail-call optimised away
     std::hint::black_box(result)
 }
+
+#[derive(Clone, Debug)]
+struct SenderNotifier {
+    tx: std::sync::mpsc::Sender<notify::Event>,
+}
+
+impl notify::Notifier for SenderNotifier {
+    fn notify(&mut self, event: notify::Event) -> std::io::Result<()> {
+        // If the sender doesn't care, neither do we
+        let _ = self.tx.send(event);
+        Ok(())
+    }
+}
diff --git a/crates/libtest2-harness/src/notify/mod.rs b/crates/libtest2-harness/src/notify/mod.rs
index bf8a70a..30b1f38 100644
--- a/crates/libtest2-harness/src/notify/mod.rs
+++ b/crates/libtest2-harness/src/notify/mod.rs
@@ -15,6 +15,8 @@ pub(crate) use summary::*;
 pub(crate) use terse::*;
 
 pub(crate) trait Notifier {
+    fn threaded(&mut self, _yes: bool) {}
+
     fn notify(&mut self, event: Event) -> std::io::Result<()>;
 }
 
diff --git a/crates/libtest2-harness/src/notify/pretty.rs b/crates/libtest2-harness/src/notify/pretty.rs
index 7d73a72..8af209b 100644
--- a/crates/libtest2-harness/src/notify/pretty.rs
+++ b/crates/libtest2-harness/src/notify/pretty.rs
@@ -7,6 +7,7 @@ use super::OK;
 #[derive(Debug)]
 pub(crate) struct PrettyRunNotifier<W> {
     writer: W,
+    is_multithreaded: bool,
     summary: super::Summary,
     name_width: usize,
 }
@@ -15,6 +16,7 @@ impl<W: std::io::Write> PrettyRunNotifier<W> {
     pub(crate) fn new(writer: W) -> Self {
         Self {
             writer,
+            is_multithreaded: false,
             summary: Default::default(),
             name_width: 0,
         }
@@ -22,6 +24,10 @@ impl<W: std::io::Write> PrettyRunNotifier<W> {
 }
 
 impl<W: std::io::Write> super::Notifier for PrettyRunNotifier<W> {
+    fn threaded(&mut self, yes: bool) {
+        self.is_multithreaded = yes;
+    }
+
     fn notify(&mut self, event: Event) -> std::io::Result<()> {
         self.summary.notify(event.clone())?;
         match event {
@@ -36,16 +42,21 @@ impl<W: std::io::Write> super::Notifier for PrettyRunNotifier<W> {
                 self.summary.write_start(&mut self.writer)?;
             }
             Event::CaseStart { name, .. } => {
-                write!(self.writer, "test {: <1$} ... ", name, self.name_width)?;
-                self.writer.flush()?;
+                if !self.is_multithreaded {
+                    write!(self.writer, "test {: <1$} ... ", name, self.name_width)?;
+                    self.writer.flush()?;
+                }
             }
-            Event::CaseComplete { status, .. } => {
+            Event::CaseComplete { name, status, .. } => {
                 let (s, style) = match status {
                     Some(RunStatus::Ignored) => ("ignored", IGNORED),
                     Some(RunStatus::Failed) => ("FAILED", FAILED),
                     None => ("ok", OK),
                 };
 
+                if self.is_multithreaded {
+                    write!(self.writer, "test {: <1$} ... ", name, self.name_width)?;
+                }
                 writeln!(self.writer, "{}{s}{}", style.render(), style.render_reset())?;
             }
             Event::SuiteComplete { .. } => {
diff --git a/crates/libtest2-mimic/Cargo.toml b/crates/libtest2-mimic/Cargo.toml
index 3a03e34..12a0585 100644
--- a/crates/libtest2-mimic/Cargo.toml
+++ b/crates/libtest2-mimic/Cargo.toml
@@ -23,9 +23,10 @@ pre-release-replacements = [
 ]
 
 [features]
-default = ["json", "junit"]
+default = ["json", "junit", "threads"]
 json = ["libtest2-harness/json"]
 junit = ["libtest2-harness/junit"]
+threads = ["libtest2-harness/threads"]
 
 [dependencies]
 libtest2-harness = { version = "0.1.0", path = "../libtest2-harness" }
diff --git a/crates/libtest2-mimic/tests/testsuite/main_thread.rs b/crates/libtest2-mimic/tests/testsuite/main_thread.rs
index 35712f2..2c4b2d1 100644
--- a/crates/libtest2-mimic/tests/testsuite/main_thread.rs
+++ b/crates/libtest2-mimic/tests/testsuite/main_thread.rs
@@ -4,12 +4,16 @@ fn check_test_on_main_thread() {
         r#"
 fn main() {
     use libtest2_mimic::Trial;
+    use libtest2_mimic::RunError;
     let outer_thread = std::thread::current().id();
     libtest2_mimic::Harness::with_env()
         .cases(vec![
             Trial::test("check", move |_| {
-                assert_eq!(outer_thread, std::thread::current().id());
-                Ok(())
+                if outer_thread == std::thread::current().id() {
+                    Ok(())
+                } else {
+                    Err(RunError::fail("thread IDs mismatch"))
+                }
             })
         ])
         .main();
diff --git a/crates/libtest2-mimic/tests/testsuite/mixed_bag.rs b/crates/libtest2-mimic/tests/testsuite/mixed_bag.rs
index 9c8968f..35eb643 100644
--- a/crates/libtest2-mimic/tests/testsuite/mixed_bag.rs
+++ b/crates/libtest2-mimic/tests/testsuite/mixed_bag.rs
@@ -87,14 +87,7 @@ test result: FAILED. 2 passed; 1 failed; 5 ignored; 0 filtered out; finished in
 "#,
         r#"
 running 8 tests
-test bear  ... ignored
-test bunny ... ignored
-test cat   ... ok
-test dog   ... FAILED
-test fly   ... ignored
-test fox   ... ok
-test frog  ... ignored
-test owl   ... ignored
+...
 
 failures: