diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -219,6 +219,7 @@
     'ArcanistGitCommitMessageHardpointQuery' => 'query/ArcanistGitCommitMessageHardpointQuery.php',
     'ArcanistGitCommitSymbolCommitHardpointQuery' => 'ref/commit/ArcanistGitCommitSymbolCommitHardpointQuery.php',
     'ArcanistGitLandEngine' => 'land/ArcanistGitLandEngine.php',
+    'ArcanistGitLocalState' => 'repository/state/ArcanistGitLocalState.php',
     'ArcanistGitUpstreamPath' => 'repository/api/ArcanistGitUpstreamPath.php',
     'ArcanistGitWorkingCopy' => 'workingcopy/ArcanistGitWorkingCopy.php',
     'ArcanistGitWorkingCopyRevisionHardpointQuery' => 'query/ArcanistGitWorkingCopyRevisionHardpointQuery.php',
@@ -401,6 +402,7 @@
     'ArcanistRepositoryAPI' => 'repository/api/ArcanistRepositoryAPI.php',
     'ArcanistRepositoryAPIMiscTestCase' => 'repository/api/__tests__/ArcanistRepositoryAPIMiscTestCase.php',
     'ArcanistRepositoryAPIStateTestCase' => 'repository/api/__tests__/ArcanistRepositoryAPIStateTestCase.php',
+    'ArcanistRepositoryLocalState' => 'repository/state/ArcanistRepositoryLocalState.php',
     'ArcanistRepositoryRef' => 'ref/ArcanistRepositoryRef.php',
     'ArcanistReusedAsIteratorXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistReusedAsIteratorXHPASTLinterRule.php',
     'ArcanistReusedAsIteratorXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistReusedAsIteratorXHPASTLinterRuleTestCase.php',
@@ -1227,6 +1229,7 @@
     'ArcanistGitCommitMessageHardpointQuery' => 'ArcanistWorkflowGitHardpointQuery',
     'ArcanistGitCommitSymbolCommitHardpointQuery' => 'ArcanistWorkflowGitHardpointQuery',
     'ArcanistGitLandEngine' => 'ArcanistLandEngine',
+    'ArcanistGitLocalState' => 'ArcanistRepositoryLocalState',
     'ArcanistGitUpstreamPath' => 'Phobject',
     'ArcanistGitWorkingCopy' => 'ArcanistWorkingCopy',
     'ArcanistGitWorkingCopyRevisionHardpointQuery' => 'ArcanistWorkflowGitHardpointQuery',
@@ -1412,6 +1415,7 @@
     'ArcanistRepositoryAPI' => 'Phobject',
     'ArcanistRepositoryAPIMiscTestCase' => 'PhutilTestCase',
     'ArcanistRepositoryAPIStateTestCase' => 'PhutilTestCase',
+    'ArcanistRepositoryLocalState' => 'Phobject',
     'ArcanistRepositoryRef' => 'ArcanistRef',
     'ArcanistReusedAsIteratorXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistReusedAsIteratorXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
diff --git a/src/repository/state/ArcanistGitLocalState.php b/src/repository/state/ArcanistGitLocalState.php
new file mode 100644
--- /dev/null
+++ b/src/repository/state/ArcanistGitLocalState.php
@@ -0,0 +1,147 @@
+<?php
+
+final class ArcanistGitLocalState
+  extends ArcanistRepositoryLocalState {
+
+  private $localCommit;
+  private $localRef;
+  private $localPath;
+
+  public function getLocalRef() {
+    return $this->localRef;
+  }
+
+  public function getLocalPath() {
+    return $this->localPath;
+  }
+
+  protected function executeSaveLocalState() {
+    $api = $this->getRepositoryAPI();
+
+    $commit = $api->getWorkingCopyRevision();
+
+    list($ref) = $api->execxLocal('rev-parse --abbrev-ref HEAD');
+    $ref = trim($ref);
+    if ($ref === 'HEAD') {
+      $ref = null;
+      $where = pht(
+        'Saving local state (at detached commit "%s").',
+        $this->getDisplayHash($commit));
+    } else {
+      $where = pht(
+        'Saving local state (on ref "%s" at commit "%s").',
+        $ref,
+        $this->getDisplayHash($commit));
+    }
+
+    $this->localRef = $ref;
+    $this->localCommit = $commit;
+
+    if ($ref !== null) {
+      $this->localPath = $api->getPathToUpstream($ref);
+    }
+
+    $log = $this->getWorkflow()->getLogEngine();
+    $log->writeStatus(pht('SAVE STATE'), $where);
+  }
+
+  protected function executeRestoreLocalState() {
+    $api = $this->getRepositoryAPI();
+
+    $log = $this->getWorkflow()->getLogEngine();
+
+    $ref = $this->localRef;
+    $commit = $this->localCommit;
+
+    if ($ref !== null) {
+      $where = pht(
+        'Restoring local state (to ref "%s" at commit "%s").',
+        $ref,
+        $this->getDisplayHash($commit));
+    } else {
+      $where = pht(
+        'Restoring local state (to detached commit "%s").',
+        $this->getDisplayHash($commit));
+    }
+
+    $log->writeStatus(pht('LOAD STATE'), $where);
+
+    if ($ref !== null) {
+      $api->execxLocal('checkout -B %s %s --', $ref, $commit);
+
+      // TODO: We save, but do not restore, the upstream configuration of
+      // this branch.
+
+    } else {
+      $api->execxLocal('checkout %s --', $commit);
+    }
+
+    $api->execxLocal('submodule update --init --recursive');
+  }
+
+  protected function executeDiscardLocalState() {
+    // We don't have anything to clean up in Git.
+    return;
+  }
+
+  protected function canStashChanges() {
+    return true;
+  }
+
+  protected function getIgnoreHints() {
+    return array(
+      pht(
+        'To configure Git to ignore certain files in this working copy, '.
+        'add the file paths to "%s".',
+        '.git/info/exclude'),
+    );
+  }
+
+  protected function saveStash() {
+    $api = $this->getRepositoryAPI();
+
+    // NOTE: We'd prefer to "git stash create" here, because using "push"
+    // and "pop" means we're affecting the stash list as a side effect.
+
+    // However, under Git 2.21.1, "git stash create" exits with no output,
+    // no error, and no effect if the working copy contains only untracked
+    // files. For now, accept mutations to the stash list.
+
+    $api->execxLocal('stash push --include-untracked --');
+
+    $log = $this->getWorkflow()->getLogEngine();
+    $log->writeStatus(
+      pht('SAVE STASH'),
+      pht('Saved uncommitted changes from working copy.'));
+
+    return true;
+  }
+
+  protected function restoreStash($stash_ref) {
+    $api = $this->getRepositoryAPI();
+
+    $log = $this->getWorkflow()->getLogEngine();
+    $log->writeStatus(
+      pht('LOAD STASH'),
+      pht('Restoring uncommitted changes to working copy.'));
+
+    // NOTE: Under Git 2.21.1, "git stash apply" does not accept "--".
+    $api->execxLocal('stash apply');
+  }
+
+  protected function discardStash($stash_ref) {
+    $api = $this->getRepositoryAPI();
+
+    // NOTE: Under Git 2.21.1, "git stash drop" does not accept "--".
+    $api->execxLocal('stash drop');
+  }
+
+  private function getDisplayStashRef($stash_ref) {
+    return substr($stash_ref, 0, 12);
+  }
+
+  private function getDisplayHash($hash) {
+    return substr($hash, 0, 12);
+  }
+
+}
diff --git a/src/repository/state/ArcanistRepositoryLocalState.php b/src/repository/state/ArcanistRepositoryLocalState.php
new file mode 100644
--- /dev/null
+++ b/src/repository/state/ArcanistRepositoryLocalState.php
@@ -0,0 +1,245 @@
+<?php
+
+abstract class ArcanistRepositoryLocalState
+  extends Phobject {
+
+  private $repositoryAPI;
+  private $shouldRestore;
+  private $stashRef;
+  private $workflow;
+
+  final public function setWorkflow(ArcanistWorkflow $workflow) {
+    $this->workflow = $workflow;
+    return $this;
+  }
+
+  final public function getWorkflow() {
+    return $this->workflow;
+  }
+
+  final public function setRepositoryAPI(ArcanistRepositoryAPI $api) {
+    $this->repositoryAPI = $api;
+    return $this;
+  }
+
+  final public function getRepositoryAPI() {
+    return $this->repositoryAPI;
+  }
+
+  final public function saveLocalState() {
+    $api = $this->getRepositoryAPI();
+
+    $working_copy_display = tsprintf(
+      "    %s: %s\n",
+      pht('Working Copy'),
+      $api->getPath());
+
+    $conflicts = $api->getMergeConflicts();
+    if ($conflicts) {
+      echo tsprintf(
+        "\n%!\n%W\n\n%s\n",
+        pht('MERGE CONFLICTS'),
+        pht('You have merge conflicts in this working copy.'),
+        $working_copy_display);
+
+      $lists = array();
+
+      $lists[] = $this->newDisplayFileList(
+        pht('Merge conflicts in working copy:'),
+        $conflicts);
+
+      $this->printFileLists($lists);
+
+      throw new PhutilArgumentUsageException(
+        pht(
+          'Resolve merge conflicts before proceeding.'));
+    }
+
+    $externals = $api->getDirtyExternalChanges();
+    if ($externals) {
+      $message = pht(
+        '%s submodule(s) have uncommitted or untracked changes:',
+        new PhutilNumber(count($externals)));
+
+      $prompt = pht(
+        'Ignore the changes to these %s submodule(s) and continue?',
+        new PhutilNumber(count($externals)));
+
+      $list = id(new PhutilConsoleList())
+        ->setWrap(false)
+        ->addItems($externals);
+
+      id(new PhutilConsoleBlock())
+        ->addParagraph($message)
+        ->addList($list)
+        ->draw();
+
+      $ok = phutil_console_confirm($prompt, $default_no = false);
+      if (!$ok) {
+        throw new ArcanistUserAbortException();
+      }
+    }
+
+    $uncommitted = $api->getUncommittedChanges();
+    $unstaged = $api->getUnstagedChanges();
+    $untracked = $api->getUntrackedChanges();
+
+    // We already dealt with externals.
+    $unstaged = array_diff($unstaged, $externals);
+
+    // We only want files which are purely uncommitted.
+    $uncommitted = array_diff($uncommitted, $unstaged);
+    $uncommitted = array_diff($uncommitted, $externals);
+
+    if ($untracked || $unstaged || $uncommitted) {
+      echo tsprintf(
+        "\n%!\n%W\n\n%s\n",
+        pht('UNCOMMITTED CHANGES'),
+        pht('You have uncommitted changes in this working copy.'),
+        $working_copy_display);
+
+      $lists = array();
+
+      $lists[] = $this->newDisplayFileList(
+        pht('Untracked changes in working copy:'),
+        $untracked);
+
+      $lists[] = $this->newDisplayFileList(
+        pht('Unstaged changes in working copy:'),
+        $unstaged);
+
+      $lists[] = $this->newDisplayFileList(
+        pht('Uncommitted changes in working copy:'),
+        $uncommitted);
+
+      $this->printFileLists($lists);
+
+      if ($untracked) {
+        $hints = $this->getIgnoreHints();
+        foreach ($hints as $hint) {
+          echo tsprintf("%?\n", $hint);
+        }
+      }
+
+      if ($this->canStashChanges()) {
+
+        $query = pht('Stash these changes and continue?');
+
+        $this->getWorkflow()
+          ->getPrompt('arc.state.stash')
+          ->setQuery($query)
+          ->execute();
+
+        $stash_ref = $this->saveStash();
+
+        if ($stash_ref === null) {
+          throw new Exception(
+            pht(
+              'Expected a non-null return from call to "%s->saveStash()".',
+              get_class($this)));
+        }
+
+        $this->stashRef = $stash_ref;
+      } else {
+        throw new PhutilArgumentUsageException(
+          pht(
+            'You can not continue with uncommitted changes. Commit or '.
+            'discard them before proceeding.'));
+      }
+    }
+
+    $this->executeSaveLocalState();
+    $this->shouldRestore = true;
+
+    return $this;
+  }
+
+  final public function restoreLocalState() {
+    $this->shouldRestore = false;
+
+    $this->executeRestoreLocalState();
+    if ($this->stashRef !== null) {
+      $this->restoreStash($this->stashRef);
+    }
+
+    return $this;
+  }
+
+  final public function discardLocalState() {
+    $this->shouldRestore = false;
+
+    $this->executeDiscardLocalState();
+    if ($this->stashRef !== null) {
+      $this->restoreStash($this->stashRef);
+      $this->discardStash($this->stashRef);
+      $this->stashRef = null;
+    }
+
+    return $this;
+  }
+
+  final public function __destruct() {
+    if ($this->shouldRestore) {
+      $this->restoreLocalState();
+    }
+
+    $this->discardLocalState();
+  }
+
+  protected function canStashChanges() {
+    return false;
+  }
+
+  protected function saveStash() {
+    throw new PhutilMethodNotImplementedException();
+  }
+
+  protected function restoreStash($ref) {
+    throw new PhutilMethodNotImplementedException();
+  }
+
+  protected function discardStash($ref) {
+    throw new PhutilMethodNotImplementedException();
+  }
+
+  abstract protected function executeSaveLocalState();
+  abstract protected function executeRestoreLocalState();
+  abstract protected function executeDiscardLocalState();
+
+  protected function getIgnoreHints() {
+    return array();
+  }
+
+  final protected function newDisplayFileList($title, array $files) {
+    if (!$files) {
+      return null;
+    }
+
+    $items = array();
+    $items[] = tsprintf("%s\n\n", $title);
+    foreach ($files as $file) {
+      $items[] = tsprintf(
+        "    %s\n",
+        $file);
+    }
+
+    return $items;
+  }
+
+  final protected function printFileLists(array $lists) {
+    $lists = array_filter($lists);
+
+    $last_key = last_key($lists);
+    foreach ($lists as $key => $list) {
+      foreach ($list as $item) {
+        echo tsprintf('%B', $item);
+      }
+      if ($key !== $last_key) {
+        echo tsprintf("\n\n");
+      }
+    }
+
+    echo tsprintf("\n");
+  }
+
+}