diff --git a/app/client/models/DocPageModel.ts b/app/client/models/DocPageModel.ts
index ce5bedeb..abf50e49 100644
--- a/app/client/models/DocPageModel.ts
+++ b/app/client/models/DocPageModel.ts
@@ -16,7 +16,7 @@ import {menu, menuDivider, menuIcon, menuItem, menuText} from 'app/client/ui2018
 import {confirmModal} from 'app/client/ui2018/modals';
 import {AsyncFlow, CancelledError, FlowRunner} from 'app/common/AsyncFlow';
 import {delay} from 'app/common/delay';
-import {OpenDocMode} from 'app/common/DocListAPI';
+import {OpenDocMode, UserOverride} from 'app/common/DocListAPI';
 import {IGristUrlState, parseUrlId, UrlIdParts} from 'app/common/gristUrls';
 import {getReconnectTimeout} from 'app/common/gutil';
 import {canEdit} from 'app/common/roles';
@@ -32,6 +32,7 @@ export interface DocInfo extends Document {
   isPreFork: boolean;
   isFork: boolean;
   isRecoveryMode: boolean;
+  userOverride: UserOverride|null;
   isBareFork: boolean;  // a document created without logging in, which is treated as a
                         // fork without an original.
   idParts: UrlIdParts;
@@ -56,6 +57,7 @@ export interface DocPageModel {
   isPrefork: Observable<boolean>;
   isFork: Observable<boolean>;
   isRecoveryMode: Observable<boolean>;
+  userOverride: Observable<UserOverride|null>;
   isBareFork: Observable<boolean>;
   isSample: Observable<boolean>;
 
@@ -93,6 +95,7 @@ export class DocPageModelImpl extends Disposable implements DocPageModel {
   public readonly isPrefork = Computed.create(this, this.currentDoc, (use, doc) => doc ? doc.isPreFork : false);
   public readonly isFork = Computed.create(this, this.currentDoc, (use, doc) => doc ? doc.isFork : false);
   public readonly isRecoveryMode = Computed.create(this, this.currentDoc, (use, doc) => doc ? doc.isRecoveryMode : false);
+  public readonly userOverride = Computed.create(this, this.currentDoc, (use, doc) => doc ? doc.userOverride : null);
   public readonly isBareFork = Computed.create(this, this.currentDoc, (use, doc) => doc ? doc.isBareFork : false);
   public readonly isSample = Computed.create(this, this.currentDoc, (use, doc) => doc ? doc.isSample : false);
 
@@ -245,8 +248,9 @@ export class DocPageModelImpl extends Disposable implements DocPageModel {
     flow.onDispose(() => comm.releaseDocConnection(doc.id));
 
     const openDocResponse = await comm.openDoc(doc.id, doc.openMode, linkParameters);
-    if (openDocResponse.recoveryMode) {
-      doc.isRecoveryMode = true;
+    if (openDocResponse.recoveryMode || openDocResponse.userOverride) {
+      doc.isRecoveryMode = Boolean(openDocResponse.recoveryMode);
+      doc.userOverride = openDocResponse.userOverride || null;
       this.currentDoc.set({...doc});
     }
     const gdModule = await gristDocModulePromise;
@@ -329,6 +333,7 @@ function buildDocInfo(doc: Document, mode: OpenDocMode): DocInfo {
     ...doc,
     isFork,
     isRecoveryMode: false,  // we don't know yet, will learn when doc is opened.
+    userOverride: null,     // ditto.
     isSample,
     isPreFork,
     isBareFork,
diff --git a/app/client/ui/TopBar.ts b/app/client/ui/TopBar.ts
index 9d915f75..4a492db8 100644
--- a/app/client/ui/TopBar.ts
+++ b/app/client/ui/TopBar.ts
@@ -44,10 +44,12 @@ export function createTopBarDoc(owner: MultiHolder, appModel: AppModel, pageMode
           docNameSave: renameDoc,
           pageNameSave: getRenamePageFn(gristDoc),
           cancelRecoveryMode: getCancelRecoveryModeFn(gristDoc),
+          cancelUserOverride: getCancelUserOverrideFn(gristDoc),
           isPageNameReadOnly: (use) => use(gristDoc.isReadonly) || typeof use(gristDoc.activeViewId) !== 'number',
           isDocNameReadOnly: (use) => use(gristDoc.isReadonly) || use(pageModel.isFork),
           isFork: pageModel.isFork,
           isRecoveryMode: pageModel.isRecoveryMode,
+          userOverride: pageModel.userOverride,
           isFiddle: Computed.create(owner, (use) => use(pageModel.isPrefork) && !use(pageModel.isSample)),
           isSnapshot: Computed.create(owner, doc, (use, _doc) => Boolean(_doc && _doc.idParts.snapshotId)),
           isPublic: Computed.create(owner, doc, (use, _doc) => Boolean(_doc && _doc.public)),
@@ -100,6 +102,15 @@ function getCancelRecoveryModeFn(gristDoc: GristDoc): () => Promise<void> {
   };
 }
 
+function getCancelUserOverrideFn(gristDoc: GristDoc): () => Promise<void> {
+  return async () => {
+    const url = new URL(window.location.href);
+    url.searchParams.delete('aclAsUser_');
+    url.searchParams.delete('aclAsUserId_');
+    window.location.assign(url.href);
+  };
+}
+
 function topBarUndoBtn(iconName: IconName, ...domArgs: DomElementArg[]): Element {
   return cssHoverCircle(
     cssTopBarUndoBtn(iconName),
diff --git a/app/client/ui2018/breadcrumbs.ts b/app/client/ui2018/breadcrumbs.ts
index c4ad9a61..a20eaa8b 100644
--- a/app/client/ui2018/breadcrumbs.ts
+++ b/app/client/ui2018/breadcrumbs.ts
@@ -9,6 +9,7 @@ import { urlState } from 'app/client/models/gristUrlState';
 import { colors, testId } from 'app/client/ui2018/cssVars';
 import { editableLabel } from 'app/client/ui2018/editableLabel';
 import { icon } from 'app/client/ui2018/icons';
+import { UserOverride } from 'app/common/DocListAPI';
 import { BindableValue, dom, Observable, styled } from 'grainjs';
 import { tooltip } from 'popweasel';
 
@@ -85,11 +86,13 @@ export function docBreadcrumbs(
     docNameSave: (val: string) => Promise<void>,
     pageNameSave: (val: string) => Promise<void>,
     cancelRecoveryMode: () => Promise<void>,
+    cancelUserOverride: () => Promise<void>,
     isDocNameReadOnly?: BindableValue<boolean>,
     isPageNameReadOnly?: BindableValue<boolean>,
     isFork: Observable<boolean>,
     isFiddle: Observable<boolean>,
     isRecoveryMode: Observable<boolean>,
+    userOverride: Observable<UserOverride|null>,
     isSnapshot?: Observable<boolean>,
     isPublic?: Observable<boolean>,
   }
@@ -118,11 +121,17 @@ export function docBreadcrumbs(
         }
         if (use(options.isRecoveryMode)) {
           return cssAlertTag('recovery mode',
-                             dom('a', dom.on('click', async () => {
-                               await options.cancelRecoveryMode()
-                             }), icon('CrossSmall')),
+                             dom('a', dom.on('click', () => options.cancelRecoveryMode()),
+                                 icon('CrossSmall')),
                              testId('recovery-mode-tag'));
         }
+        const userOverride = use(options.userOverride);
+        if (userOverride) {
+          return cssAlertTag(userOverride.user?.email || 'override',
+                             dom('a', dom.on('click', () => options.cancelUserOverride()),
+                                 icon('CrossSmall')),
+                             testId('user-override-tag'));
+        }
         if (use(options.isFiddle)) {
           return cssTag('fiddle', tooltip({title: fiddleExplanation}), testId('fiddle-tag'));
         }
diff --git a/app/common/DocListAPI.ts b/app/common/DocListAPI.ts
index 493ce5a9..ce96df27 100644
--- a/app/common/DocListAPI.ts
+++ b/app/common/DocListAPI.ts
@@ -1,7 +1,9 @@
 import {ActionGroup} from 'app/common/ActionGroup';
 import {TableDataAction} from 'app/common/DocActions';
 import {LocalPlugin} from 'app/common/plugin';
+import {Role} from 'app/common/roles';
 import {StringUnion} from 'app/common/StringUnion';
+import {FullUser} from 'app/common/UserAPI';
 
 // Possible flavors of items in a list of documents.
 export type DocEntryTag = ''|'sample'|'invite'|'shared';
@@ -43,6 +45,12 @@ export interface OpenLocalDocResult {
   log: ActionGroup[];
   plugins: LocalPlugin[];
   recoveryMode?: boolean;
+  userOverride?: UserOverride;
+}
+
+export class UserOverride {
+  user: FullUser|null;
+  access: Role|null;
 }
 
 export interface DocListAPI {
diff --git a/app/gen-server/lib/HomeDBManager.ts b/app/gen-server/lib/HomeDBManager.ts
index a146378a..68304f66 100644
--- a/app/gen-server/lib/HomeDBManager.ts
+++ b/app/gen-server/lib/HomeDBManager.ts
@@ -359,7 +359,7 @@ export class HomeDBManager extends EventEmitter {
   }
 
   public getUser(userId: number): Promise<User|undefined> {
-    return User.findOne(userId);
+    return User.findOne(userId, {relations: ["logins"]});
   }
 
   public async getFullUser(userId: number): Promise<FullUser> {
diff --git a/app/server/lib/ActiveDoc.ts b/app/server/lib/ActiveDoc.ts
index 17299924..1b97aa00 100644
--- a/app/server/lib/ActiveDoc.ts
+++ b/app/server/lib/ActiveDoc.ts
@@ -160,6 +160,10 @@ export class ActiveDoc extends EventEmitter {
 
   public get recoveryMode(): boolean { return this._recoveryMode; }
 
+  public async getUserOverride(docSession: OptDocSession) {
+    return this._granularAccess.getUserOverride(docSession);
+  }
+
   // Helpers to log a message along with metadata about the request.
   public logDebug(s: OptDocSession, msg: string, ...args: any[]) { this._log('debug', s, msg, ...args); }
   public logInfo(s: OptDocSession, msg: string, ...args: any[]) { this._log('info', s, msg, ...args); }
@@ -417,7 +421,7 @@ export class ActiveDoc extends EventEmitter {
     await this._actionHistory.initialize();
     this._granularAccess = new GranularAccess(this.docData, (query) => {
       return this._fetchQueryFromDB(query, false);
-    }, this.recoveryMode);
+    }, this.recoveryMode, this._docManager.getHomeDbManager(), this.docName);
     await this._granularAccess.update();
     this._sharing = new Sharing(this, this._actionHistory, this._modificationLock);
 
diff --git a/app/server/lib/DocManager.ts b/app/server/lib/DocManager.ts
index ec1b21c5..e5564b9a 100644
--- a/app/server/lib/DocManager.ts
+++ b/app/server/lib/DocManager.ts
@@ -54,6 +54,10 @@ export class DocManager extends EventEmitter {
     this._homeDbManager = dbManager;
   }
 
+  public getHomeDbManager() {
+    return this._homeDbManager;
+  }
+
   /**
    * Returns an implementation of the DocListAPI for the given Client object.
    */
@@ -303,6 +307,7 @@ export class DocManager extends EventEmitter {
       log: recentActions,
       plugins: activeDoc.docPluginManager.getPlugins(),
       recoveryMode: activeDoc.recoveryMode,
+      userOverride: await activeDoc.getUserOverride(docSession),
     };
   }
 
diff --git a/app/server/lib/GranularAccess.ts b/app/server/lib/GranularAccess.ts
index 8e459dbb..1be5fcbf 100644
--- a/app/server/lib/GranularAccess.ts
+++ b/app/server/lib/GranularAccess.ts
@@ -12,14 +12,18 @@ import { RemoveRecord, ReplaceTableData, UpdateRecord } from 'app/common/DocActi
 import { CellValue, ColValues, DocAction, getTableId, isSchemaAction } from 'app/common/DocActions';
 import { TableDataAction, UserAction } from 'app/common/DocActions';
 import { DocData } from 'app/common/DocData';
+import { UserOverride } from 'app/common/DocListAPI';
 import { ErrorWithCode } from 'app/common/ErrorWithCode';
 import { AclMatchInput, InfoView } from 'app/common/GranularAccessClause';
 import { RuleSet, UserInfo } from 'app/common/GranularAccessClause';
 import { getSetMapValue, isObject } from 'app/common/gutil';
-import { canView } from 'app/common/roles';
+import { canView, Role } from 'app/common/roles';
+import { FullUser } from 'app/common/UserAPI';
+import { HomeDBManager } from 'app/gen-server/lib/HomeDBManager';
 import { compileAclFormula } from 'app/server/lib/ACLFormula';
 import { getDocSessionAccess, getDocSessionUser, OptDocSession } from 'app/server/lib/DocSession';
 import * as log from 'app/server/lib/log';
+import { integerParam } from 'app/server/lib/requestUtils';
 import { getRelatedRows, getRowIdsFromDocAction } from 'app/server/lib/RowAccess';
 import cloneDeep = require('lodash/cloneDeep');
 import get = require('lodash/get');
@@ -114,7 +118,9 @@ export class GranularAccess {
   public constructor(
     private _docData: DocData,
     private _fetchQueryFromDB: (query: Query) => Promise<TableDataAction>,
-    private _recoveryMode: boolean) {
+    private _recoveryMode: boolean,
+    private _homeDbManager: HomeDBManager | null,
+    private _docId: string) {
   }
 
   /**
@@ -488,6 +494,11 @@ export class GranularAccess {
     this._filterColumns(data[3], (colId) => permInfo.getColumnAccess(tableId, colId).read !== 'deny');
   }
 
+  public async getUserOverride(docSession: OptDocSession): Promise<UserOverride|undefined> {
+    await this._getUser(docSession);
+    return this._getUserAttributes(docSession).override;
+  }
+
   /**
    * Strip out any denied columns from an action.  Returns null if nothing is left.
    * accessFn may throw if denials are fatal.
@@ -799,9 +810,36 @@ export class GranularAccess {
    * created by user-attribute rules.
    */
   private async _getUser(docSession: OptDocSession): Promise<UserInfo> {
-    const access = getDocSessionAccess(docSession);
-    const fullUser = getDocSessionUser(docSession);
+    const linkParameters = docSession.authorizer?.getLinkParameters() || {};
+    let access: Role | null;
+    let fullUser: FullUser | null;
     const attrs = this._getUserAttributes(docSession);
+    access = getDocSessionAccess(docSession);
+
+    // If aclAsUserId/aclAsUser is set, then override user for acl purposes.
+    if (linkParameters.aclAsUserId || linkParameters.aclAsUser) {
+      if (!this.isOwner(docSession)) { throw new Error('only an owner can override user'); }
+      if (attrs.override) {
+        // Used cached properties.
+        access = attrs.override.access;
+        fullUser = attrs.override.user;
+      } else {
+        // Look up user information in database.
+        if (!this._homeDbManager) { throw new Error('database required'); }
+        const user = linkParameters.aclAsUserId ?
+          (await this._homeDbManager.getUser(integerParam(linkParameters.aclAsUserId))) :
+          (await this._homeDbManager.getUserByLogin(linkParameters.aclAsUser));
+        const docAuth = user && await this._homeDbManager.getDocAuthCached({
+          urlId: this._docId,
+          userId: user.id
+        });
+        access = docAuth?.access || null;
+        fullUser = user && this._homeDbManager.makeFullUser(user) || null;
+        attrs.override = { access, user: fullUser };
+      }
+    } else {
+      fullUser = getDocSessionUser(docSession);
+    }
     const user: UserInfo = {};
     user.Access = access;
     user.UserID = fullUser?.id || null;
@@ -809,7 +847,7 @@ export class GranularAccess {
     user.Name = fullUser?.name || null;
     // If viewed from a websocket, collect any link parameters included.
     // TODO: could also get this from rest api access, just via a different route.
-    user.Link = docSession.authorizer?.getLinkParameters() || {};
+    user.Link = linkParameters;
     // Include origin info if accessed via the rest api.
     // TODO: could also get this for websocket access, just via a different route.
     user.Origin = docSession.req?.get('origin') || null;
@@ -1114,6 +1152,7 @@ class EmptyRecordView implements InfoView {
  */
 class UserAttributes {
   public rows: {[clauseName: string]: InfoView} = {};
+  public override?: UserOverride;
 }
 
 // A function for extracting one of the create/read/update/delete/schemaEdit permissions