diff --git a/src/endpoint_monitor/store.py b/src/endpoint_monitor/store.py
new file mode 100644
index 0000000..b3efa63
--- /dev/null
+++ b/src/endpoint_monitor/store.py
@@ -0,0 +1,80 @@
+import asyncio
+from datetime import UTC, datetime
+from uuid import UUID, uuid4
+
+from .models import CheckResult, Monitor, MonitorCreate, MonitorStatus, MonitorUpdate
+
+
+class StoreError(Exception):
+    pass
+
+
+class NotFound(StoreError):
+    pass
+
+
+class Conflict(StoreError):
+    pass
+
+
+class MonitorStore:
+    def __init__(self, max_monitors: int) -> None:
+        self._items: dict[UUID, Monitor] = {}
+        self._lock = asyncio.Lock()
+        self._max = max_monitors
+
+    async def create(self, data: MonitorCreate) -> Monitor:
+        async with self._lock:
+            if len(self._items) >= self._max:
+                raise Conflict("monitor capacity reached")
+            if any(item.name == data.name for item in self._items.values()):
+                raise Conflict("monitor name already exists")
+            now = datetime.now(UTC)
+            item = Monitor(id=uuid4(), name=data.name, url=data.url, created_at=now, updated_at=now, revision=1)
+            self._items[item.id] = item
+            return item.model_copy(deep=True)
+
+    async def list(self) -> list[Monitor]:
+        async with self._lock:
+            return [item.model_copy(deep=True) for item in self._items.values()]
+
+    async def get(self, monitor_id: UUID) -> Monitor:
+        async with self._lock:
+            item = self._items.get(monitor_id)
+            if item is None:
+                raise NotFound("monitor not found")
+            return item.model_copy(deep=True)
+
+    async def update(self, monitor_id: UUID, data: MonitorUpdate) -> Monitor:
+        async with self._lock:
+            item = self._items.get(monitor_id)
+            if item is None:
+                raise NotFound("monitor not found")
+            changes = data.model_dump(exclude_unset=True)
+            if "name" in changes and changes["name"] is None:
+                changes.pop("name")
+            if "url" in changes and changes["url"] is None:
+                changes.pop("url")
+            name = changes.get("name")
+            if name and any(other.id != monitor_id and other.name == name for other in self._items.values()):
+                raise Conflict("monitor name already exists")
+            if "url" in changes and changes["url"] != item.url:
+                changes.update(status=MonitorStatus.UNKNOWN, last_check=None)
+            changes.update(updated_at=datetime.now(UTC), revision=item.revision + 1)
+            item = item.model_copy(update=changes)
+            self._items[monitor_id] = item
+            return item.model_copy(deep=True)
+
+    async def delete(self, monitor_id: UUID) -> None:
+        async with self._lock:
+            if self._items.pop(monitor_id, None) is None:
+                raise NotFound("monitor not found")
+
+    async def record_check(self, monitor_id: UUID, result: CheckResult) -> Monitor:
+        async with self._lock:
+            item = self._items.get(monitor_id)
+            if item is None:
+                raise NotFound("monitor not found")
+            item = item.model_copy(update={"status": result.status, "last_check": result, "updated_at": datetime.now(UTC), "revision": item.revision + 1})
+            self._items[monitor_id] = item
+            return item.model_copy(deep=True)