diff --git a/packages/catalog-client/package.json b/packages/catalog-client/package.json
index 4119614503..03addce840 100644
--- a/packages/catalog-client/package.json
+++ b/packages/catalog-client/package.json
@@ -50,6 +50,7 @@
   "dependencies": {
     "@backstage/catalog-model": "workspace:^",
     "@backstage/errors": "workspace:^",
+    "@backstage/types": "workspace:^",
     "cross-fetch": "^4.0.0",
     "uri-template": "^2.0.0"
   },
diff --git a/packages/catalog-client/src/testUtils/InMemoryCatalogClient.ts b/packages/catalog-client/src/testUtils/InMemoryCatalogClient.ts
index 745273c6bc..a975a79b80 100644
--- a/packages/catalog-client/src/testUtils/InMemoryCatalogClient.ts
+++ b/packages/catalog-client/src/testUtils/InMemoryCatalogClient.ts
@@ -17,7 +17,9 @@
 import {
   AddLocationRequest,
   AddLocationResponse,
+  CATALOG_FILTER_EXISTS,
   CatalogApi,
+  EntityFilterQuery,
   GetEntitiesByRefsRequest,
   GetEntitiesByRefsResponse,
   GetEntitiesRequest,
@@ -34,9 +36,72 @@ import {
 import {
   CompoundEntityRef,
   Entity,
+  parseEntityRef,
   stringifyEntityRef,
 } from '@backstage/catalog-model';
 import { NotFoundError, NotImplementedError } from '@backstage/errors';
+import { JsonObject, JsonValue } from '@backstage/types';
+
+function pick(obj: JsonObject, key: string): JsonValue | undefined {
+  const parts = key.split('.');
+
+  return parts.reduce((acc, part, index) => {
+    if (!acc) {
+      return acc;
+    }
+    if (typeof acc === 'object' && acc !== null && !Array.isArray(acc)) {
+      const value = acc[part];
+      if (value) {
+        return value;
+      }
+      const rest = parts.slice(index).join('.');
+      const restValue = acc[rest];
+      if (restValue) {
+        return restValue;
+      }
+    }
+    return undefined;
+  }, obj as JsonValue | undefined);
+}
+
+function filterCompare(
+  expected: (string | symbol) | (string | symbol)[],
+  value: JsonValue,
+) {
+  const expect = (Array.isArray(expected) ? expected : [expected]).map(e =>
+    e.toString().toLocaleLowerCase('en-US'),
+  );
+  return expect.includes(String(value).toLocaleLowerCase('en-US'));
+}
+
+function createFilter(
+  filterOrFilters?: EntityFilterQuery,
+): (entity: Entity) => boolean {
+  if (!filterOrFilters) {
+    return () => true;
+  }
+  const filters = Array.isArray(filterOrFilters)
+    ? filterOrFilters
+    : [filterOrFilters];
+
+  return entity => {
+    return filters.some(filter => {
+      for (const [key, expectedValue] of Object.entries(filter)) {
+        const entityValue = pick(entity, key);
+        if (entityValue === undefined) {
+          return false;
+        }
+        if (expectedValue === CATALOG_FILTER_EXISTS) {
+          continue;
+        }
+        if (!filterCompare(expectedValue, entityValue)) {
+          return false;
+        }
+      }
+      return true;
+    });
+  };
+}
 
 /**
  * Implements a VERY basic fake catalog client that stores entities in memory.
@@ -47,78 +112,106 @@ import { NotFoundError, NotImplementedError } from '@backstage/errors';
  */
 export class InMemoryCatalogClient implements CatalogApi {
   #entities: Entity[];
+  #entitiesByRef: Map<string, Entity>;
 
   constructor(options?: { entities?: Entity[] }) {
     this.#entities = options?.entities?.slice() ?? [];
+    this.#entitiesByRef = new Map(
+      this.#entities.map(e => [stringifyEntityRef(e), e]),
+    );
   }
 
   async getEntities(
-    _request?: GetEntitiesRequest,
+    request?: GetEntitiesRequest | undefined,
   ): Promise<GetEntitiesResponse> {
-    // TODO(freben): Fields, filters etc
-    return { items: this.#entities.slice() };
+    const filter = createFilter(request?.filter);
+    return { items: this.#entities.filter(filter) };
   }
 
   async getEntitiesByRefs(
     request: GetEntitiesByRefsRequest,
   ): Promise<GetEntitiesByRefsResponse> {
-    const entitiesByRef = new Map<string, Entity>();
-    for (const entity of this.#entities) {
-      entitiesByRef.set(stringifyEntityRef(entity), entity);
-    }
-
-    const items = request.entityRefs.map(candidateRef =>
-      entitiesByRef.get(candidateRef),
-    );
-
-    // TODO(freben): Fields, filters etc
-    return { items };
+    const filter = createFilter(request.filter);
+    return {
+      items: request.entityRefs
+        .map(ref => this.#entitiesByRef.get(ref))
+        .filter(e => (e ? filter(e) : true)),
+    };
   }
 
   async queryEntities(
-    _request?: QueryEntitiesRequest,
+    request?: QueryEntitiesRequest | undefined,
   ): Promise<QueryEntitiesResponse> {
-    // TODO(freben): Fields, filters etc
+    if (request && 'cursor' in request) {
+      return { items: [], pageInfo: {}, totalItems: 0 };
+    }
+    const filter = createFilter(request?.filter);
+    // TODO(Rugvip): Pagination
     return {
-      items: this.#entities.slice(),
-      totalItems: this.#entities.length,
+      items: this.#entities.filter(filter),
       pageInfo: {},
+      totalItems: this.#entities.length,
     };
   }
 
   async getEntityAncestors(
     request: GetEntityAncestorsRequest,
   ): Promise<GetEntityAncestorsResponse> {
-    const entity = this.#entities.find(
-      e => stringifyEntityRef(e) === request.entityRef,
-    );
+    const entity = this.#entitiesByRef.get(request.entityRef);
     if (!entity) {
       throw new NotFoundError(`Entity with ref ${request.entityRef} not found`);
     }
     return {
-      rootEntityRef: request.entityRef,
       items: [{ entity, parentEntityRefs: [] }],
+      rootEntityRef: request.entityRef,
     };
   }
 
   async getEntityByRef(
     entityRef: string | CompoundEntityRef,
   ): Promise<Entity | undefined> {
-    const ref =
-      typeof entityRef === 'string' ? entityRef : stringifyEntityRef(entityRef);
-    return this.#entities.find(e => stringifyEntityRef(e) === ref);
+    return this.#entitiesByRef.get(
+      stringifyEntityRef(parseEntityRef(entityRef)),
+    );
   }
 
   async removeEntityByUid(uid: string): Promise<void> {
-    this.#entities = this.#entities.filter(e => e.metadata.uid !== uid);
+    const index = this.#entities.findIndex(e => e.metadata.uid === uid);
+    if (index !== -1) {
+      const entity = this.#entities[index];
+      this.#entitiesByRef.delete(stringifyEntityRef(entity));
+      this.#entities.splice(index, 1);
+    }
   }
 
   async refreshEntity(_entityRef: string): Promise<void> {}
 
   async getEntityFacets(
-    _request: GetEntityFacetsRequest,
+    request: GetEntityFacetsRequest,
   ): Promise<GetEntityFacetsResponse> {
-    throw new NotImplementedError('Method not implemented.');
+    const filter = createFilter(request.filter);
+    const filteredEntities = this.#entities.filter(filter);
+    const facets = Object.fromEntries(
+      request.facets.map(facet => {
+        const facetValues = new Map<string, number>();
+        for (const entity of filteredEntities) {
+          const value = pick(entity, facet);
+          if (value) {
+            facetValues.set(
+              String(value),
+              (facetValues.get(String(value)) ?? 0) + 1,
+            );
+          }
+        }
+        const counts = Array.from(facetValues.entries()).map(
+          ([value, count]) => ({ value, count }),
+        );
+        return [facet, counts];
+      }),
+    );
+    return {
+      facets,
+    };
   }
 
   async getLocationById(_id: string): Promise<Location | undefined> {
diff --git a/yarn.lock b/yarn.lock
index 03cb6f421c..9aec0defac 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -3840,6 +3840,7 @@ __metadata:
     "@backstage/catalog-model": "workspace:^"
     "@backstage/cli": "workspace:^"
     "@backstage/errors": "workspace:^"
+    "@backstage/types": "workspace:^"
     cross-fetch: ^4.0.0
     msw: ^1.0.0
     uri-template: ^2.0.0