diff --git a/src/labthings_fastapi/computed_properties.py b/src/labthings_fastapi/computed_properties.py
new file mode 100644
index 00000000..c7cb7a6c
--- /dev/null
+++ b/src/labthings_fastapi/computed_properties.py
@@ -0,0 +1,192 @@
+"""Computed properties.
+
+A computed property is like a functional property, but it only depends on
+properties which are observable.
+This means LabThings is able to recompute
+it whenever its dependencies change, allowing it to be observed.
+"""
+
+from collections.abc import Callable
+from typing import TYPE_CHECKING, Any, Generic, cast
+
+from anyio import create_memory_object_stream
+from anyio.streams.memory import MemoryObjectSendStream
+
+from labthings_fastapi.exceptions import PropertyNotObservableError
+from labthings_fastapi.message_broker import Message
+from labthings_fastapi.properties import FunctionalProperty, Owner, Value
+
+if TYPE_CHECKING:
+    from labthings_fastapi.thing import Thing
+
+
+class AccessWrapper:
+    """Wrap access to the properties of an object."""
+
+    def __init__(self, obj: "Thing", dependencies: set[str]) -> None:
+        """Initialise the AccessWrapper.
+
+        :param obj: the object being wrapped.
+        :param dependencies: a set to use for tracking dependencies.
+        """
+        self._obj = obj
+        self._dependencies = dependencies
+
+    def __getattr__(self, name: str) -> Any:
+        """Proxy attribute access to the underlying object.
+
+        :param name: The name of the attribute being accessed.
+        :return: The value of the object.
+        :raises PropertyNotObservableError: if a non-observable property
+            is accessed.
+        :raises TypeError: if something other than a property is accessed.
+        """
+        if name in self._obj.properties:
+            if self._obj.properties[name].is_observable:
+                self._dependencies.add(name)
+                print(f"Found an access to {name}")
+                return getattr(self._obj, name)
+            else:
+                raise PropertyNotObservableError(
+                    f"{self._obj.name}.{name} is not observable. "
+                    "Only observable properties may be accessed from a "
+                    "computed property."
+                )
+        else:
+            raise TypeError(
+                f"{self._obj.name}.{name} is not a property. "
+                "Only observable properties may be accessed from a "
+                "computed property."
+            )
+
+    def __setattr__(self, name: str, value: Any) -> None:
+        """Don't allow attributes to be set, there should be no side-effects.
+
+        :param name: the name of the attribute.
+        :param value: the value to set.
+        :raises AttributeError: because the wrapper is read-only.
+        """
+        raise AttributeError("Computed properties may not set values.")
+
+
+def access_wrapper(obj: Owner, dependencies: set[str]) -> Owner:
+    """Wrap a Thing to record attribute access.
+
+    This function is preferred to instantiating AccessWrapper directly,
+    as it ensures the wrapper is type hinted as the original object.
+
+    :param obj: the Thing to wrap.
+    :param dependencies: a set to store dependencies.
+    :return: `obj` with an attribute access wrapper.
+    """
+    # Typing note: AccessWrapper proxies attribute access back to the
+    # wrapped object, so its signature should be identical to `obj`
+    # and thus the `cast` below is justified.
+    wrapper = AccessWrapper(obj, dependencies=dependencies)
+    return cast(Owner, wrapper)
+
+
+class ComputedProperty(FunctionalProperty[Owner, Value], Generic[Owner, Value]):
+    """A property that recomputes its value on demand.
+
+    This is a way to make functional properties observable.
+    """
+
+    @property
+    def is_computed(self) -> bool:
+        """Whether the property is a computed property."""
+        return True
+
+    def instance_get(self, obj: Owner, dependencies: set[str] | None = None) -> Value:
+        """Get the value of this functional property.
+
+        :param obj: the object on which this property is being accessed.
+        :param dependencies: an optional set to be populated with properties accessed
+            during recomputation.
+        :return: the value of the property.
+        """
+        if dependencies is not None:
+            wrapper = access_wrapper(obj, dependencies=dependencies)
+            return self._fget(wrapper)
+        else:
+            return self._fget(obj)
+
+    async def _watch_for_changes(self, obj: Owner) -> None:
+        """Watch for changes in our dependencies, and recompute as needed.
+
+        This method will evaluate the computed property, tracking which
+        properties are accessed. It then subscribes to these properties,
+        and will recompute the property as required.
+
+        :param obj: the object on which the property is defined.
+        """
+        send, recv = create_memory_object_stream[Message](max_buffer_size=1)
+        # Subscribe to a non-existent affordance, to ensure that the stream
+        # is closed by the message broker even if there are no other
+        # subscriptions.
+        obj._thing_server_interface.subscribe(obj.name, "#dummy", send)
+
+        dependencies: set[str] = set()
+        # Add a message to the stream, so we initialise everything immediately
+        # in the `async for` loop.
+        initial_message = Message(obj.name, self.name, "property", None)
+        await send.send(initial_message)
+
+        # Whenever a dependency changes, we'll recompute and publish an update
+        async for message in recv:
+            old_dependencies = dependencies
+            dependencies = set()
+            value = self.instance_get(obj, dependencies=dependencies)
+            await self._update_subscriptions(obj, dependencies, old_dependencies, send)
+            if message is initial_message:
+                continue
+            obj._thing_server_interface.publish(
+                Message(obj.name, self.name, "property", value)
+            )
+
+    @staticmethod
+    async def _update_subscriptions(
+        obj: Owner,
+        dependencies: set[str],
+        old_dependencies: set[str],
+        send_stream: MemoryObjectSendStream[Message],
+    ) -> None:
+        """Subscribe and unsubscribe to other properties.
+
+        :param obj: the Thing on which we are defined.
+        :param dependencies: a set of the names of current dependencies.
+        :param old_dependencies: previous dependencies - any not in ``dependencies``
+            will be unsubscribed.
+        :param send_stream: the stream to use for subscriptions.
+        """
+        # The streams exist and a coroutine is monitoring them. Now, subscribe
+        # to changes in our dependencies
+        for affordance in dependencies:
+            obj._thing_server_interface.subscribe(obj.name, affordance, send_stream)
+        # Unsubscribe from any dependencies no longer needed
+        for affordance in old_dependencies.difference(dependencies):
+            obj._thing_server_interface.unsubscribe(obj.name, affordance, send_stream)
+
+
+def computed_property(fget: Callable[[Owner], Value]) -> ComputedProperty[Owner, Value]:
+    """Decorate a method as a computed property.
+
+    :param fget: the getter function, which must depend only on observable properties.
+    :return: a computed property descriptor.
+    """
+    return ComputedProperty(fget=fget)
+
+
+def initialise_computed_properties(thing: "Thing") -> None:
+    """Initialise the computed properties on a Thing.
+
+    :param thing: the Thing on which to initialise computed properties.
+    """
+    for prop in thing.properties.values():
+        if prop.is_computed:
+            computed_property = prop.get_descriptor()
+            if not isinstance(computed_property, ComputedProperty):
+                continue
+            thing._thing_server_interface.start_async_task_soon(
+                computed_property._watch_for_changes, thing
+            )
diff --git a/src/labthings_fastapi/properties.py b/src/labthings_fastapi/properties.py
index 2de6fc68..de715f15 100644
--- a/src/labthings_fastapi/properties.py
+++ b/src/labthings_fastapi/properties.py
@@ -414,6 +414,11 @@ def __init__(
     `False` for functional properties.
     """
 
+    @builtins.property
+    def is_computed(self) -> bool:  # noqa: DOC201
+        """Whether the property is a computed property."""
+        return False
+
     @staticmethod
     def _validate_constraints(constraints: Mapping[str, Any]) -> FieldConstraints:
         """Validate an untyped dictionary of constraints.
@@ -1214,6 +1219,30 @@ def reset(self) -> None:
         """
         return self.get_descriptor().reset(self.owning_object_or_error())
 
+    @builtins.property
+    def is_computed(self) -> bool:  # noqa: DOC201
+        """Whether the property is a computed property."""
+        # This is done by inspecting the name rather than an isinstance check
+        # to avoid circular dependencies.
+        return self.get_descriptor().is_computed
+
+    def publish(self) -> None:
+        """Get the property's value, and publish it to any observers.
+
+        This reduces boilerplate when you are managing property notifications manually,
+        and is also used to initialise computed properties.
+        """
+        value = self.get()
+        obj = self.owning_object_or_error()
+        obj._thing_server_interface.publish(
+            Message(
+                thing=obj.name,
+                affordance=self.name,
+                message_type="property",
+                payload=value,
+            )
+        )
+
     def validate(self, value: Any) -> Value:
         """Use the validation logic in `self.model`.
 
diff --git a/src/labthings_fastapi/thing_server_interface.py b/src/labthings_fastapi/thing_server_interface.py
index 3acbf011..29418e0f 100644
--- a/src/labthings_fastapi/thing_server_interface.py
+++ b/src/labthings_fastapi/thing_server_interface.py
@@ -18,6 +18,8 @@
 )
 from weakref import ReferenceType, ref
 
+from anyio.streams.memory import MemoryObjectSendStream
+
 from labthings_fastapi.exceptions import FeatureNotEnabledError, ServerNotRunningError
 from labthings_fastapi.global_lock import GlobalLock
 from labthings_fastapi.message_broker import Message
@@ -156,6 +158,40 @@ def publish(self, message: Message) -> None:
         except ServerNotRunningError:
             pass  # If the server isn't running yet, we can't publish events.
 
+    def subscribe(
+        self, thing: str, affordance: str, stream: MemoryObjectSendStream[Message]
+    ) -> None:
+        """Subscribe to messages from an affordance.
+
+        :param thing: the name of the Thing being subscribed to.
+        :param affordance: the name of the affordance being subscribed to.
+        :param stream: the stream messages should be sent to.
+        :raises ServerNotRunningError: if the server hasn't started yet. It doesn't
+            make sense to subscribe to events if the event loop isn't yet running.
+        """
+        try:
+            broker = self._get_server().message_broker
+            self.start_async_task_soon(broker.subscribe, thing, affordance, stream)
+        except ServerNotRunningError:
+            raise
+
+    def unsubscribe(
+        self, thing: str, affordance: str, stream: MemoryObjectSendStream[Message]
+    ) -> None:
+        """Unsubscribe to messages from an affordance.
+
+        :param thing: the name of the Thing being subscribed to.
+        :param affordance: the name of the affordance being subscribed to.
+        :param stream: the stream messages should be sent to.
+        :raises ServerNotRunningError: if the server hasn't started yet. It doesn't
+            make sense to unsubscribe from events if the event loop isn't yet running.
+        """
+        try:
+            broker = self._get_server().message_broker
+            self.start_async_task_soon(broker.unsubscribe, thing, affordance, stream)
+        except ServerNotRunningError:
+            raise
+
     @property
     def settings_folder(self) -> str:
         """The path to a folder where persistent files may be saved."""
diff --git a/tests/test_computed_properties.py b/tests/test_computed_properties.py
new file mode 100644
index 00000000..bdaa62f7
--- /dev/null
+++ b/tests/test_computed_properties.py
@@ -0,0 +1,60 @@
+"""Test computed properties in isolation."""
+
+from typing import Literal
+
+import pytest
+
+import labthings_fastapi as lt
+from labthings_fastapi.computed_properties import computed_property
+from labthings_fastapi.exceptions import PropertyNotObservableError
+from labthings_fastapi.testing import create_thing_without_server
+
+
+class MyThing(lt.Thing):
+    quantity: int = lt.property(default=0)
+    quantity2: int = lt.property(default=0)
+    selector: Literal["q1", "q2"] = lt.property(default="q1")
+
+    @computed_property
+    def double(self) -> int:
+        return 2 * self.quantity
+
+    @computed_property
+    def selected(self) -> int:
+        if self.selector == "q1":
+            return self.quantity
+        elif self.selector == "q2":
+            return self.quantity2
+        else:
+            raise ValueError("invalid selector")
+
+    @lt.property
+    def unobservable(self) -> str:
+        return "unobservable"
+
+    @computed_property
+    def broken(self) -> str:
+        return self.unobservable
+
+
+def test_dependencies():
+    thing = create_thing_without_server(MyThing)
+
+    thing.quantity = 1
+    assert thing.double == 2
+    assert MyThing.double._dependencies[thing] == {"quantity"}
+
+    assert thing.selected == 1
+    assert MyThing.selected._dependencies[thing] == {"selector", "quantity"}
+
+    thing.selector = "q2"
+    thing.quantity2 = 10
+    assert thing.selected == 10
+    assert MyThing.selected._dependencies[thing] == {"selector", "quantity2"}
+
+
+def test_unobservable():
+    """Check computed properties error if they depend on unobservable quantities."""
+    thing = create_thing_without_server(MyThing)
+    with pytest.raises(PropertyNotObservableError):
+        _ = thing.broken