diff --git a/README.md b/README.md
index 4ba670616..b4a621dcc 100644
--- a/README.md
+++ b/README.md
@@ -138,10 +138,10 @@ From the root of the repository, run:
 
 ```
 # Starts browser-navigable web page
-yarn run start:test-server:dev
+yarn run start:e2e:mocha
 
 # Starts backend servers
-yarn run test:e2e:dev-setup
+yarn run start:e2e:docker
 ```
 
 Then choose between the following run options:
diff --git a/package.json b/package.json
index c4618ee33..1ff18c725 100644
--- a/package.json
+++ b/package.json
@@ -11,6 +11,8 @@
     "bootstrap": "lerna bootstrap",
     "start": "lerna run start --parallel",
     "test:unit": "lerna run test:unit --parallel",
+    "start:e2e:mocha": "yarn run start:test-server:dev",
+    "start:e2e:docker": "yarn run test:e2e:dev-setup",
     "start:test-server": "node e2e-server.js",
     "start:test-server:dev": "node e2e-server.js --dev",
     "test:e2e": "bash test.sh stable",
diff --git a/packages/snjs/lib/application.ts b/packages/snjs/lib/application.ts
index 2aff2dadf..ce7c7a3cb 100644
--- a/packages/snjs/lib/application.ts
+++ b/packages/snjs/lib/application.ts
@@ -1,3 +1,4 @@
+import { TagNoteCountChangeObserver } from './protocol/collection/tag_notes_index';
 import { TransactionalMutation } from './services/item_manager';
 import { FeatureStatus } from '@Lib/services/features_service';
 import { Settings } from './services/settings_service';
@@ -857,6 +858,20 @@ export class SNApplication {
     return this.itemManager.notesMatchingSmartTag(smartTag);
   }
 
+  public addNoteCountChangeObserver(
+    observer: TagNoteCountChangeObserver
+  ): () => void {
+    return this.itemManager.addNoteCountChangeObserver(observer);
+  }
+
+  public allCountableNotesCount(): number {
+    return this.itemManager.allCountableNotesCount();
+  }
+
+  public countableNotesForTag(tag: SNTag | SNSmartTag): number {
+    return this.itemManager.countableNotesForTag(tag);
+  }
+
   /** Returns an item's direct references */
   public referencesForItem(item: SNItem, contentType?: ContentType): SNItem[] {
     let references = this.itemManager.referencesForItem(item.uuid);
diff --git a/packages/snjs/lib/models/app/note.ts b/packages/snjs/lib/models/app/note.ts
index 6fb26702d..5d636984d 100644
--- a/packages/snjs/lib/models/app/note.ts
+++ b/packages/snjs/lib/models/app/note.ts
@@ -1,7 +1,8 @@
-import { PayloadContent } from '@Payloads/generator';
-import { PayloadFormat } from './../../protocol/payloads/formats';
+import { ContentType } from '@Lib/index';
 import { isNullOrUndefined } from '@Lib/utils';
 import { AppDataField, ItemMutator, SNItem } from '@Models/core/item';
+import { PayloadContent } from '@Payloads/generator';
+import { PayloadFormat } from './../../protocol/payloads/formats';
 import { PurePayload } from './../../protocol/payloads/pure_payload';
 
 export interface NoteContent extends PayloadContent {
@@ -14,6 +15,9 @@ export interface NoteContent extends PayloadContent {
   spellcheck?: boolean;
 }
 
+export const isNote = (x: SNItem): x is SNNote =>
+  x.content_type === ContentType.Note;
+
 /** A note item */
 export class SNNote extends SNItem implements NoteContent {
   public readonly title!: string;
diff --git a/packages/snjs/lib/models/app/tag.ts b/packages/snjs/lib/models/app/tag.ts
index 68cd0f82d..8346d3e02 100644
--- a/packages/snjs/lib/models/app/tag.ts
+++ b/packages/snjs/lib/models/app/tag.ts
@@ -9,6 +9,9 @@ export interface TagContent extends ItemContent {
   title: string;
 }
 
+export const isTag = (x: SNItem): x is SNTag =>
+  x.content_type === ContentType.Tag;
+
 /**
  * Allows organization of notes into groups.
  * A tag can have many notes, and a note can have many tags.
@@ -23,7 +26,7 @@ export class SNTag extends SNItem implements TagContent {
 
   get noteReferences(): ContentReference[] {
     const references = this.payload.safeReferences;
-    return references.filter(ref => ref.content_type === ContentType.Note)
+    return references.filter((ref) => ref.content_type === ContentType.Note);
   }
 
   get noteCount(): number {
diff --git a/packages/snjs/lib/protocol/collection/indexes.ts b/packages/snjs/lib/protocol/collection/indexes.ts
new file mode 100644
index 000000000..d51e67981
--- /dev/null
+++ b/packages/snjs/lib/protocol/collection/indexes.ts
@@ -0,0 +1,12 @@
+import { SNItem } from '../../models/core/item';
+
+export interface ItemDelta {
+  changed: SNItem[];
+  inserted: SNItem[];
+  discarded: SNItem[];
+  ignored: SNItem[];
+}
+
+export interface SNIndex {
+  onChange(delta: ItemDelta): void;
+}
diff --git a/packages/snjs/lib/protocol/collection/item_collection.ts b/packages/snjs/lib/protocol/collection/item_collection.ts
index 0b5d31cd7..e8d28c251 100644
--- a/packages/snjs/lib/protocol/collection/item_collection.ts
+++ b/packages/snjs/lib/protocol/collection/item_collection.ts
@@ -3,6 +3,7 @@ import { compareValues, isNullOrUndefined, uniqueArrayByKey } from '@Lib/utils';
 import { SNItem } from './../../models/core/item';
 import { ContentType } from '@standardnotes/common';
 import { UuidString } from './../../types';
+import { ItemDelta, SNIndex } from './indexes';
 
 export enum CollectionSort {
   CreatedAt = 'created_at',
@@ -13,7 +14,9 @@ export type SortDirection = 'asc' | 'dsc';
 
 /** The item collection class builds on mutable collection by providing an option to keep
  * items sorted and filtered. */
-export class ItemCollection extends MutableCollection<SNItem> {
+export class ItemCollection
+  extends MutableCollection<SNItem>
+  implements SNIndex {
   private displaySortBy: Partial<
     Record<
       ContentType,
@@ -134,9 +137,12 @@ export class ItemCollection extends MutableCollection<SNItem> {
       const previousElement = !isNullOrUndefined(previousIndex)
         ? sortedElements[previousIndex]
         : undefined;
-      /** If the element is deleted, or if it no longer exists in the primary map (because
+
+      /**
+       *  If the element is deleted, or if it no longer exists in the primary map (because
        * it was discarded without neccessarily being marked as deleted), it does not pass
-       * the filter. If no filter the element passes by default. */
+       * the filter. If no filter the element passes by default.
+       */
       const passes =
         element.deleted || !this.map[element.uuid]
           ? false
@@ -260,4 +266,14 @@ export class ItemCollection extends MutableCollection<SNItem> {
     }
     this.sortedMap[contentType] = cleaned;
   }
+
+  public onChange(delta: ItemDelta): void {
+    const changedOrInserted = delta.changed.concat(delta.inserted);
+
+    if (changedOrInserted.length > 0) {
+      this.set(changedOrInserted);
+    }
+
+    this.discard(delta.discarded);
+  }
 }
diff --git a/packages/snjs/lib/protocol/collection/item_collection_notes_view.ts b/packages/snjs/lib/protocol/collection/item_collection_notes_view.ts
index b253b0b55..d03187f71 100644
--- a/packages/snjs/lib/protocol/collection/item_collection_notes_view.ts
+++ b/packages/snjs/lib/protocol/collection/item_collection_notes_view.ts
@@ -1,7 +1,8 @@
+import { ContentType } from '@Lib/index';
+import { SNNote, SNTag } from '../../models';
 import { SNSmartTag } from './../../models/app/smartTag';
+import { ItemDelta, SNIndex } from './indexes';
 import { ItemCollection } from './item_collection';
-import { SNNote, SNTag } from '../../models';
-import { ContentType } from '@standardnotes/common';
 import {
   criteriaForSmartTag,
   NotesDisplayCriteria,
@@ -11,7 +12,7 @@ import {
 /**
  * A view into ItemCollection that allows filtering by tag and smart tag.
  */
-export class ItemCollectionNotesView {
+export class ItemCollectionNotesView implements SNIndex {
   private displayedNotes: SNNote[] = [];
   private needsRebuilding = true;
 
@@ -35,7 +36,22 @@ export class ItemCollectionNotesView {
     return notesMatchingCriteria(criteria, this.collection);
   }
 
+  public displayElements(): SNNote[] {
+    if (this.needsRebuilding) {
+      this.rebuildList();
+    }
+    return this.displayedNotes.slice();
+  }
+
   private rebuildList(): void {
+    this.displayedNotes = notesMatchingCriteria(
+      this.currentCriteria,
+      this.collection
+    );
+    this.needsRebuilding = false;
+  }
+
+  private get currentCriteria(): NotesDisplayCriteria {
     const mostRecentVersionOfTags = this.criteria.tags
       .map((tag) => {
         if (tag.isSystemSmartTag) {
@@ -45,21 +61,15 @@ export class ItemCollectionNotesView {
         }
       })
       .filter((tag) => tag != undefined);
+
     const criteria = NotesDisplayCriteria.Copy(this.criteria, {
       tags: mostRecentVersionOfTags,
     });
-    this.displayedNotes = notesMatchingCriteria(criteria, this.collection);
-  }
 
-  setNeedsRebuilding() {
-    this.needsRebuilding = true;
+    return criteria;
   }
 
-  displayElements() {
-    if (this.needsRebuilding) {
-      this.rebuildList();
-      this.needsRebuilding = false;
-    }
-    return this.displayedNotes.slice();
+  public onChange(_delta: ItemDelta): void {
+    this.needsRebuilding = true;
   }
 }
diff --git a/packages/snjs/lib/protocol/collection/tag_notes_index.ts b/packages/snjs/lib/protocol/collection/tag_notes_index.ts
new file mode 100644
index 000000000..0a2c4798d
--- /dev/null
+++ b/packages/snjs/lib/protocol/collection/tag_notes_index.ts
@@ -0,0 +1,113 @@
+import { removeFromArray } from '@Lib/utils';
+import { ItemCollection } from './item_collection';
+import { SNNote } from '@Lib/models';
+import { SNTag } from '@Lib/index';
+import { UuidString } from '@Lib/types';
+import { isNote } from '@Lib/models/app/note';
+import { isTag } from '@Lib/models/app/tag';
+import { ItemDelta, SNIndex } from './indexes';
+
+/** tagUuid undefined signifies all notes count change */
+export type TagNoteCountChangeObserver = (
+  tagUuid: UuidString | undefined
+) => void;
+
+export class TagNotesIndex implements SNIndex {
+  private tagToNotesMap: Partial<Record<UuidString, Set<UuidString>>> = {};
+  private allCountableNotes = new Set<UuidString>();
+  private observers: TagNoteCountChangeObserver[] = [];
+
+  constructor(private collection: ItemCollection) {}
+
+  private isNoteCountable = (note: SNNote) => {
+    return !note.archived && !note.trashed;
+  };
+
+  public addCountChangeObserver(
+    observer: TagNoteCountChangeObserver
+  ): () => void {
+    this.observers.push(observer);
+
+    return () => {
+      removeFromArray(this.observers, observer);
+    };
+  }
+
+  private notifyObservers(tagUuid: UuidString | undefined) {
+    for (const observer of this.observers) {
+      observer(tagUuid);
+    }
+  }
+
+  public allCountableNotesCount(): number {
+    return this.allCountableNotes.size;
+  }
+
+  public countableNotesForTag(tag: SNTag): number {
+    return this.tagToNotesMap[tag.uuid]?.size || 0;
+  }
+
+  public onChange(delta: ItemDelta): void {
+    const changedOrInserted = delta.changed.concat(delta.inserted);
+    const notes = changedOrInserted.filter(isNote);
+    const tags = changedOrInserted.filter(isTag);
+
+    this.receiveNoteChanges(notes);
+    this.receiveTagChanges(tags);
+  }
+
+  private receiveTagChanges(tags: SNTag[]): void {
+    for (const tag of tags) {
+      const uuids = tag.noteReferences.map((ref) => ref.uuid);
+      const countableUuids = uuids.filter((uuid) =>
+        this.allCountableNotes.has(uuid)
+      );
+      const previousSet = this.tagToNotesMap[tag.uuid];
+      this.tagToNotesMap[tag.uuid] = new Set(countableUuids);
+
+      if (previousSet?.size !== countableUuids.length) {
+        this.notifyObservers(tag.uuid);
+      }
+    }
+  }
+
+  private receiveNoteChanges(notes: SNNote[]): void {
+    for (const note of notes) {
+      const isCountable = this.isNoteCountable(note);
+      const previousAllCount = this.allCountableNotes.size;
+      if (isCountable) {
+        this.allCountableNotes.add(note.uuid);
+      } else {
+        this.allCountableNotes.delete(note.uuid);
+      }
+      if (previousAllCount !== this.allCountableNotes.size) {
+        this.notifyObservers(undefined);
+      }
+
+      const associatedTagUuids = this.collection.uuidsThatReferenceUuid(
+        note.uuid
+      );
+      for (const tagUuid of associatedTagUuids) {
+        const set = this.setForTag(tagUuid);
+        const previousCount = set.size;
+        if (isCountable) {
+          set.add(note.uuid);
+        } else {
+          set.delete(note.uuid);
+        }
+        if (previousCount !== set.size) {
+          this.notifyObservers(tagUuid);
+        }
+      }
+    }
+  }
+
+  private setForTag(uuid: UuidString): Set<UuidString> {
+    let set = this.tagToNotesMap[uuid];
+    if (!set) {
+      set = new Set();
+      this.tagToNotesMap[uuid] = set;
+    }
+    return set;
+  }
+}
diff --git a/packages/snjs/lib/services/item_manager.spec.ts b/packages/snjs/lib/services/item_manager.spec.ts
index a7f7ad2e3..bb4fed17f 100644
--- a/packages/snjs/lib/services/item_manager.spec.ts
+++ b/packages/snjs/lib/services/item_manager.spec.ts
@@ -1,5 +1,5 @@
 import { ItemManager, SNItem } from '@Lib/index';
-import { SNNote } from '@Lib/models';
+import { SNNote, NoteMutator } from '@Lib/models';
 import { SmartTagPredicateContent, SNSmartTag } from '@Lib/models/app/smartTag';
 import { Uuid } from '@Lib/uuid';
 import { SNTag, TagMutator } from '@Models/app/tag';
@@ -334,6 +334,59 @@ describe('itemManager', () => {
     });
   });
 
+  describe('tags notes index', () => {
+    it('counts countable notes', async () => {
+      itemManager = createService();
+
+      const parentTag = createTag('parent');
+      const childTag = createTag('child');
+      await itemManager.insertItems([parentTag, childTag]);
+      await itemManager.setTagParent(parentTag, childTag);
+
+      const parentNote = createNote('parentNote');
+      const childNote = createNote('childNote');
+      await itemManager.insertItems([parentNote, childNote]);
+
+      await itemManager.addTagToNote(parentNote, parentTag);
+      await itemManager.addTagToNote(childNote, childTag);
+
+      expect(itemManager.countableNotesForTag(parentTag)).toBe(1);
+      expect(itemManager.countableNotesForTag(childTag)).toBe(1);
+      expect(itemManager.allCountableNotesCount()).toBe(2);
+    });
+
+    it('archiving a note should update count index', async () => {
+      itemManager = createService();
+
+      const tag1 = createTag('tag 1');
+      await itemManager.insertItems([tag1]);
+
+      const note1 = createNote('note 1');
+      const note2 = createNote('note 2');
+      await itemManager.insertItems([note1, note2]);
+
+      await itemManager.addTagToNote(note1, tag1);
+      await itemManager.addTagToNote(note2, tag1);
+
+      expect(itemManager.countableNotesForTag(tag1)).toBe(2);
+      expect(itemManager.allCountableNotesCount()).toBe(2);
+
+      await itemManager.changeItem<NoteMutator>(note1.uuid, (m) => {
+        m.archived = true;
+      });
+
+      expect(itemManager.allCountableNotesCount()).toBe(1);
+      expect(itemManager.countableNotesForTag(tag1)).toBe(1);
+
+      await itemManager.changeItem<NoteMutator>(note1.uuid, (m) => {
+        m.archived = false;
+      });
+
+      expect(itemManager.allCountableNotesCount()).toBe(2);
+      expect(itemManager.countableNotesForTag(tag1)).toBe(2);
+    });
+  });
+
   describe('tags and smart tags', () => {
     it('lets me create a smart tag', async () => {
       itemManager = createService();
diff --git a/packages/snjs/lib/services/item_manager.ts b/packages/snjs/lib/services/item_manager.ts
index 8caf50e4f..1ee4514bb 100644
--- a/packages/snjs/lib/services/item_manager.ts
+++ b/packages/snjs/lib/services/item_manager.ts
@@ -1,4 +1,5 @@
 import { createMutatorForItem } from '@Lib/models/mutator';
+import { ItemDelta } from '@Lib/protocol/collection/indexes';
 import { ItemCollectionNotesView } from '@Lib/protocol/collection/item_collection_notes_view';
 import { NotesDisplayCriteria } from '@Lib/protocol/collection/notes_display_criteria';
 import { PureService } from '@Lib/services/pure_service';
@@ -15,7 +16,7 @@ import {
   ItemCollection,
   SortDirection,
 } from '@Protocol/collection/item_collection';
-import { ContentType } from '@standardnotes/common'
+import { ContentType } from '@standardnotes/common';
 import { ComponentMutator } from './../models/app/component';
 import {
   ActionsExtensionMutator,
@@ -35,6 +36,10 @@ import {
 import { TagMutator } from './../models/app/tag';
 import { ItemMutator, MutationType, SNItem } from './../models/core/item';
 import { SNPredicate } from './../models/core/predicate';
+import {
+  TagNoteCountChangeObserver,
+  TagNotesIndex,
+} from './../protocol/collection/tag_notes_index';
 import {
   PayloadContent,
   PayloadOverride,
@@ -69,6 +74,9 @@ export type TransactionalMutation = {
   mutationType?: MutationType;
 };
 
+export const isTagOrNote = (x: SNItem): x is SNNote | SNTag =>
+  x.content_type === ContentType.Note || x.content_type === ContentType.Tag;
+
 /**
  * The item manager is backed by the Payload Manager. Think of the item manager as a
  * more user-friendly or item-specific interface to creating and updating data.
@@ -85,6 +93,7 @@ export class ItemManager extends PureService {
   private collection!: ItemCollection;
   private notesView!: ItemCollectionNotesView;
   private systemSmartTags: SNSmartTag[];
+  private tagNotesIndex!: TagNotesIndex;
 
   constructor(private payloadManager: PayloadManager) {
     super();
@@ -130,6 +139,7 @@ export class ItemManager extends PureService {
       'asc'
     );
     this.notesView = new ItemCollectionNotesView(this.collection);
+    this.tagNotesIndex = new TagNotesIndex(this.collection);
   }
 
   public setDisplayOptions(
@@ -253,6 +263,29 @@ export class ItemManager extends PureService {
     ) as SNComponent[];
   }
 
+  public addNoteCountChangeObserver(
+    observer: TagNoteCountChangeObserver
+  ): () => void {
+    return this.tagNotesIndex.addCountChangeObserver(observer);
+  }
+
+  public allCountableNotesCount(): number {
+    return this.tagNotesIndex.allCountableNotesCount();
+  }
+
+  public countableNotesForTag(tag: SNTag | SNSmartTag): number {
+    if (tag.isSmartTag) {
+      if (tag.isAllTag) {
+        return this.tagNotesIndex.allCountableNotesCount();
+      }
+
+      throw Error(
+        'countableNotesForTag is not meant to be used for smart tags.'
+      );
+    }
+    return this.tagNotesIndex.countableNotesForTag(tag);
+  }
+
   public addObserver(
     contentType: ContentType | ContentType[],
     callback: ObserverCallback
@@ -301,23 +334,25 @@ export class ItemManager extends PureService {
     source: PayloadSource,
     sourceKey?: string
   ) {
-    const changedItems = changed.map((p) => CreateItemFromPayload(p));
-    const insertedItems = inserted.map((p) => CreateItemFromPayload(p));
-    const ignoredItems = ignored.map((p) => CreateItemFromPayload(p));
-    const changedOrInserted = changedItems.concat(insertedItems);
-    if (changedOrInserted.length > 0) {
-      this.collection.set(changedOrInserted);
-    }
-    const discardedItems = discarded.map((p) => CreateItemFromPayload(p));
-    for (const item of discardedItems) {
-      this.collection.discard(item);
-    }
-    this.notesView.setNeedsRebuilding();
+    const createItems = (items: PurePayload[]) =>
+      items.map((item) => CreateItemFromPayload(item));
+
+    const delta: ItemDelta = {
+      changed: createItems(changed),
+      inserted: createItems(inserted),
+      discarded: createItems(discarded),
+      ignored: createItems(ignored),
+    };
+
+    this.collection.onChange(delta);
+    this.notesView.onChange(delta);
+    this.tagNotesIndex.onChange(delta);
+
     this.notifyObservers(
-      changedItems,
-      insertedItems,
-      discardedItems,
-      ignoredItems,
+      delta.changed,
+      delta.inserted,
+      delta.discarded,
+      delta.ignored,
       source,
       sourceKey
     );