diff --git a/equinox/__init__.py b/equinox/__init__.py
index 1e5b37b8..671aff69 100644
--- a/equinox/__init__.py
+++ b/equinox/__init__.py
@@ -25,6 +25,7 @@
     branched_error_if as branched_error_if,
     EquinoxTracetimeError as EquinoxTracetimeError,
     error_if as error_if,
+    warn_if as warn_if,
 )
 from ._eval_shape import filter_eval_shape as filter_eval_shape
 from ._filters import (
diff --git a/equinox/_errors.py b/equinox/_errors.py
index 0ffb4cf0..2d5ac2a0 100644
--- a/equinox/_errors.py
+++ b/equinox/_errors.py
@@ -326,3 +326,110 @@ def assert_dce(
     else:
         # Don't run if not JIT'ing, as without the compiler nothing will be DCE'd.
         return x
+
+
+# ------------------------------------
+
+
+def warn_if(
+    x: PyTree,
+    pred: Bool[ArrayLike, "..."],
+    msg: str,
+    *,
+    category: Warning | None = None,
+    stacklevel: int = 1
+) -> PyTree:
+    """Surfaces a warning based on runtime values. Works even under JIT.
+
+    **Arguments:**
+
+    - `x`: will be returned unchanged. This is used to determine where the warning check
+        happens in the overall computation: it will happen after `x` is computed and
+        before the return value is used. `x` can be any PyTree, and it must contain at
+        least one array.
+    - `pred`: a boolean for whether to raise an warning. Can be an array of bools; an
+        warning will be raised if any of them are `True`. If vmap'd then an warning will be
+        raised if any batch element has `True`.
+    - `msg`: the string to display as an warning message.
+
+    **Returns:**
+
+    The original argument `x` unchanged. **If this return value is unused then the warning
+    check will not be performed.** (It will be removed as part of dead code
+    elimination.)
+
+    !!! Example
+
+        ```python
+        @jax.jit
+        def f(x):
+            x = warn_if(x, x < 0, "x must be >= 0")
+            # ...use x in your computation...
+            return x
+
+        f(jax.numpy.array(-1))
+        ```
+    """
+    return branched_warn_if(x, pred, 0, [msg], category=category, stacklevel=stacklevel)
+
+
+def branched_warn_if(
+    x: PyTree,
+    pred: Bool[ArrayLike, "..."],
+    index: Int[ArrayLike, "..."],
+    msgs: Sequence[str],
+    *,
+    category: Warning | None = None,
+    stacklevel: int = 1
+) -> PyTree:
+    """As [`equinox.warn_if`][], but will raise one of
+    several `msgs` depending on the value of `index`. If `index` is vmap'd, then the
+    warn message from the largest value (across the whole batch) will be used.
+    """
+    leaves = jtu.tree_leaves((x, pred, index))
+    # This carefully does not perform any JAX operations if `pred` and `index` are
+    # a bool and an int.
+    # This ensures we can use `warn_if` before init_google.
+    if any(is_array(leaf) for leaf in leaves):
+        return branched_warn_if_impl_jit(x, pred, index, msgs, category=category, stacklevel=stacklevel)
+    else:
+        return branched_warn_if_impl(x, pred, index, msgs, category=category, stacklevel=stacklevel)
+
+
+def warning_callback(message, *, category: Warning | None, stacklevel: int):
+    warnings.warn(message, category=category, stacklevel=stacklevel)
+
+
+def branched_warn_if_impl(
+    x: PyTree,
+    pred: Bool[ArrayLike, "..."],
+    index: Int[ArrayLike, "..."],
+    msgs: Sequence[str],
+    *,
+    category: Warning | None = None,
+    stacklevel: int = 1
+) -> PyTree:
+    with jax.ensure_compile_time_eval():
+        # This carefully does not perform any JAX operations if `pred` and `index` are
+        # a bool and an int.
+        # This ensures we can use `error_if` before init_google.
+        if not isinstance(pred, bool):
+            pred = unvmap_any(pred)
+        if not isinstance(index, int):
+            index = unvmap_max(index)
+        if not isinstance(pred, jax.core.Tracer):
+            if isinstance(pred, Array):
+                pred = pred.item()
+            assert type(pred) is bool
+            if pred:
+                if not isinstance(index, jax.core.Tracer):
+                    if isinstance(index, Array):
+                        index = index.item()
+                    assert type(index) is int
+                    jax.debug.callback(warning_callback, msgs[index], category=category, stacklevel=stacklevel)
+    return x
+
+
+# filter_jit does some work to produce nicer runtime warning messages.
+# We also place it here to ensure a consistent experience when using JAX in eager mode.
+branched_warn_if_impl_jit = filter_jit(branched_warn_if_impl)
\ No newline at end of file
diff --git a/tests/test_warns.py b/tests/test_warns.py
new file mode 100644
index 00000000..a3dd7e8a
--- /dev/null
+++ b/tests/test_warns.py
@@ -0,0 +1,128 @@
+import equinox as eqx
+import equinox.internal as eqxi
+import jax
+import jax.numpy as jnp
+import pytest
+import warnings
+
+
+def _f(x):
+    x = eqx.warn_if(x, x < 0, "x must be non-negative")
+    return jax.nn.relu(x)
+
+
+# Strangely, JAX raises different errors depending on context.
+_warn = pytest.warns(UserWarning)
+
+
+def test_basic():
+    jf = jax.jit(_f)
+    _f(1.0)
+    jf(1.0)
+    with _warn:
+        _f(-1.0)
+    with _warn:
+        jf(-1.0)
+    with _warn:
+        jf(-1.0)
+
+
+def test_vmap():
+    vf = jax.vmap(_f)
+    jvf = jax.jit(vf)
+    good = jnp.array([1.0, 1.0])
+    bad1 = jnp.array([1.0, -1.0])
+    bad2 = jnp.array([-1.0, -1.0])
+
+    vf(good)
+    jvf(good)
+    with _warn:
+        vf(bad1)
+    with _warn:
+        vf(bad2)
+    with _warn:
+        jvf(bad1)
+    with _warn:
+        jvf(bad2)
+
+
+def test_jvp():
+    def g(p, t):
+        return jax.jvp(_f, (p,), (t,))
+
+    jg = jax.jit(g)
+
+    for h in (g, jg):
+        h(1.0, 1.0)
+        h(1.0, -1.0)
+        with _warn:
+            h(-1.0, 1.0)
+        with _warn:
+            h(-1.0, -1.0)
+
+
+def test_grad():
+    g = jax.grad(_f)
+    jg = jax.jit(g)
+
+    for h in (g, jg):
+        h(1.0)
+        with _warn:
+            h(-1.0)
+
+
+def test_grad2():
+    @jax.jit
+    @jax.grad
+    def f(x, y, z):
+        x = eqxi.nondifferentiable_backward(x)
+        x, y = eqx.warn_if((x, y), z, "oops")
+        return y
+
+    f(1.0, 1.0, True)
+
+
+def test_tracetime():
+    @jax.jit
+    def f(x):
+        return eqx.warn_if(x, True, "hi")
+
+    with pytest.warns(UserWarning):
+        f(1.0)
+
+
+def test_assert_dce():
+    @jax.jit
+    def f(x):
+        x = x + 1
+        eqxi.assert_dce(x, msg="oh no")
+        return x
+
+    f(1.0)
+
+    @jax.jit
+    def g(x):
+        x = x + 1
+        eqxi.assert_dce(x, msg="oh no")
+        return x
+
+    with jax.disable_jit():
+        g(1.0)
+
+
+# def test_traceback_runtime_eqx():
+#     @eqx.filter_jit
+#     def f(x):
+#         return g(x)
+
+#     @eqx.filter_jit
+#     def g(x):
+#         return eqx.warn_if(x, x > 0, "egads")
+
+#     f(jnp.array(1.0))
+#     except Exception as e:
+#         assert e.__cause__ is None
+#         msg = str(e).strip()
+#         assert msg.startswith("egads")
+#         assert "EQX_ON_warn" in msg
+#         assert msg.endswith("information.")