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
@@ -397,6 +397,9 @@
     'ArcanistPHPShortTagXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistPHPShortTagXHPASTLinterRuleTestCase.php',
     'ArcanistPaamayimNekudotayimSpacingXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistPaamayimNekudotayimSpacingXHPASTLinterRule.php',
     'ArcanistPaamayimNekudotayimSpacingXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistPaamayimNekudotayimSpacingXHPASTLinterRuleTestCase.php',
+    'ArcanistPackageDescriptor' => 'packages/ArcanistPackageDescriptor.php',
+    'ArcanistPackagesLoader' => 'packages/ArcanistPackagesLoader.php',
+    'ArcanistPackagesWorkflow' => 'workflow/ArcanistPackagesWorkflow.php',
     'ArcanistParentMemberReferenceXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistParentMemberReferenceXHPASTLinterRule.php',
     'ArcanistParentMemberReferenceXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistParentMemberReferenceXHPASTLinterRuleTestCase.php',
     'ArcanistParenthesesSpacingXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistParenthesesSpacingXHPASTLinterRule.php',
@@ -1454,6 +1457,9 @@
     'ArcanistPHPShortTagXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistPaamayimNekudotayimSpacingXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistPaamayimNekudotayimSpacingXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
+    'ArcanistPackageDescriptor' => 'Phobject',
+    'ArcanistPackagesLoader' => 'Phobject',
+    'ArcanistPackagesWorkflow' => 'ArcanistWorkflow',
     'ArcanistParentMemberReferenceXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistParentMemberReferenceXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistParenthesesSpacingXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
diff --git a/src/packages/ArcanistPackageDescriptor.php b/src/packages/ArcanistPackageDescriptor.php
new file mode 100644
--- /dev/null
+++ b/src/packages/ArcanistPackageDescriptor.php
@@ -0,0 +1,98 @@
+<?php
+
+final class ArcanistPackageDescriptor
+  extends Phobject {
+
+  private $identifier;
+  private $version;
+  private $configSource;
+  private $description;
+
+  public static function fromConfig(
+    array $config_item,
+    ArcanistConfigurationSource $config_source) {
+
+    $ident = idx($config_item, 'identifier');
+    $version = idx($config_item, 'version');
+    if (!strlen($ident) || !strlen($version)) {
+      throw new ArcanistUsageException(
+        pht(
+          'Package configuration requires both identifier and a version. '.
+          '(reading %s)',
+          $config_source->getSourceDisplayName()));
+    }
+
+    return id(new self())
+      ->setIdentifier($ident)
+      ->setVersion($version)
+      ->setConfigSource($config_source);
+  }
+
+  public static function fromDirectory($directory) {
+    $filename = $directory.'/.arcpackage';
+    if (!Filesystem::pathExists($filename)) {
+      return null;
+    }
+    $manifest = phutil_json_decode(Filesystem::readFile($filename));
+    return self::fromManifest($manifest);
+  }
+
+  /**
+   * "Manifest" refers to the .arcpackage file.
+   */
+  public static function fromManifest(array $manifest) {
+    $ident = idx($manifest, 'identifier');
+    $version = idx($manifest, 'version');
+    if (!strlen($ident) || !strlen($version)) {
+      throw new ArcanistUsageException(pht('Bad manifest file!'));
+    }
+    return id(new self())
+      ->setIdentifier($ident)
+      ->setVersion($version)
+      ->setDescription(idx($manifest, 'description'));
+  }
+
+  public function __toString() {
+    return pht('Package %s (ver %s)', $this->identifier, $this->version);
+  }
+
+  public function getDirectoryName() {
+    return $this->identifier.'-'.$this->version;
+  }
+
+  public function setConfigSource(ArcanistConfigurationSource $config_source) {
+    $this->configSource = $config_source;
+    return $this;
+  }
+
+  public function getConfigSource() {
+    return $this->configSource;
+  }
+
+  public function setDescription($description) {
+    $this->description = $description;
+    return $this;
+  }
+
+  public function getDescription() {
+    return $this->description;
+  }
+
+  public function setIdentifier(string $identifier) {
+    $this->identifier = $identifier;
+    return $this;
+  }
+
+  public function getIdentifier() {
+    return $this->identifier;
+  }
+
+  public function setVersion(string $version) {
+    $this->version = $version;
+    return $this;
+  }
+
+  public function getVersion() {
+    return $this->version;
+  }
+}
diff --git a/src/packages/ArcanistPackagesLoader.php b/src/packages/ArcanistPackagesLoader.php
new file mode 100644
--- /dev/null
+++ b/src/packages/ArcanistPackagesLoader.php
@@ -0,0 +1,152 @@
+<?php
+
+final class ArcanistPackagesLoader
+  extends Phobject {
+
+  private $configurationSourceList;
+
+  public function setConfigurationSourceList(
+    ArcanistConfigurationSourceList $sources) {
+
+    $this->configurationSourceList = $sources;
+    return $this;
+  }
+
+  public function loadPackages() {
+    $log = new ArcanistLogEngine();
+    $requested = $this->listRequestedPackages();
+
+    $this->checkForConflicts($requested);
+
+    $missing = array();
+
+    foreach ($requested as $package) {
+      $package_dir = $this->resolvePath($package);
+      $found = ArcanistPackageDescriptor::fromDirectory($package_dir);
+      if ($found != null) {
+        if ($found->getIdentifier() != $package->getIdentifier() ||
+            $found->getVersion() != $package->getVersion()) {
+          $log->writeError(
+            pht('PACKAGE LOAD ERROR'),
+            pht(
+              'Expected to find package %s in location %s, found %s instead',
+              $package->getIdentifier(),
+              $package_dir,
+              $found->getIdentifier()));
+          $missing[] = $package;
+          continue;
+        }
+
+        try {
+          $this->loadPackage($found);
+        } catch (PhutilBootloaderException $ex) {
+          $log->writeError(
+            pht('LOAD ERROR'),
+            pht(
+              'Failed to load package "%s". This package is '.
+              'specified by "%s"',
+              $package->getIdentifier(),
+              $package->getConfigurationSource()->getSourceDisplayName()));
+
+          $prompt = pht('Continue loading without this package?');
+          if (!phutil_console_confirm($prompt)) {
+            throw $ex;
+          }
+        }
+      } else {
+        $missing[] = $package;
+      }
+    }
+
+    if ($missing) {
+      $log->writeError(
+        pht('MISSING PACKAGES'),
+        pht(
+          'The following packages were requested but not found: %s.',
+          implode(', ', $missing)));
+
+        // TODO try to install; use `prompts`?
+        $prompt = pht('Continue without these packages?');
+        if (!phutil_console_confirm($prompt)) {
+          throw new ArcanistUserAbortException('Requested package not found.');
+        }
+    }
+  }
+
+  private function checkForConflicts($requested) {
+    $log = new ArcanistLogEngine();
+    $packages = mgroup($requested, 'getIdentifier', 'getVersion');
+
+    foreach ($packages as $identifier => $versions) {
+      if (count($versions) > 1) {
+        $requests = array();
+        foreach ($versions as $version => $list) {
+          $package = head($list);
+          $requests[] = pht(
+            'v%s - Requested from %s',
+            $package->getVersion(),
+            $package->getConfigSource()->getSourceDisplayName());
+        }
+
+        $log->writeError(
+          pht('CONFLICTING PACKAGE REQUESTED'),
+          pht(
+            'The package "%s" has conflicting requirements: %s.',
+            $identifier,
+            implode('; ', $requests)));
+
+        throw new ArcanistUsageException(
+          pht('Conflicting requests for package %s', $identifier));
+      }
+    }
+  }
+
+  private function loadPackage(ArcanistPackageDescriptor $package) {
+    phutil_load_library($this->resolvePath($package).'/src');
+  }
+
+  public function listRequestedPackages() {
+    $requested = array();
+    $items = $this->configurationSourceList->getStorageValueList('packages');
+
+    foreach ($items as $item) {
+      foreach ($item->getValue() as $package_item) {
+        $requested[] = ArcanistPackageDescriptor::fromConfig(
+          $package_item,
+          $item->getConfigurationSource());
+      }
+    }
+    return $requested;
+  }
+
+  /**
+   * This method may be slow, because it looks for and loads lots of files.
+   */
+  public function listInstalledPackages() {
+    $packages_root = $this->getPackagesDirectory();
+
+    if (!Filesystem::pathExists($packages_root)) {
+      return array();
+    }
+
+    $dirs = Filesystem::listDirectory($packages_root);
+
+    $packages = array();
+    foreach ($dirs as $dir) {
+      $packages[$dir] =
+        ArcanistPackageDescriptor::fromDirectory($packages_root.'/'.$dir);
+    }
+    return array_filter($packages);
+  }
+
+  private function resolvePath(ArcanistPackageDescriptor $package) {
+    return $this->getPackagesDirectory().'/'.$package->getDirectoryName();
+  }
+
+  private function getPackagesDirectory() {
+    if (phutil_is_windows()) {
+      return getenv('APPDATA').'/.cache/arcanist-packages';
+    }
+    return getenv('HOME').'/.cache/arcanist-packages';
+  }
+}
diff --git a/src/runtime/ArcanistRuntime.php b/src/runtime/ArcanistRuntime.php
--- a/src/runtime/ArcanistRuntime.php
+++ b/src/runtime/ArcanistRuntime.php
@@ -102,6 +102,7 @@
     $config = $config_engine->newConfigurationSourceList();
 
     $this->loadLibraries($config_engine, $config, $args);
+    $this->loadPackages($config_engine, $config, $args);
 
     // Now that we've loaded libraries, we can validate configuration.
     // Do this before continuing since configuration can impact other
@@ -371,11 +372,11 @@
           break;
       }
 
-      $this->loadLibrary($engine, $library_source, $description);
+      $this->loadLegacyLibrary($engine, $library_source, $description);
     }
   }
 
-  private function loadLibrary(
+  private function loadLegacyLibrary(
     ArcanistConfigurationEngine $engine,
     $location,
     $description) {
@@ -512,6 +513,16 @@
     }
   }
 
+  private function loadPackages(
+    ArcanistConfigurationEngine $engine,
+    ArcanistConfigurationSourceList $config,
+    PhutilArgumentParser $args) {
+
+    $loader = id(new ArcanistPackagesLoader())
+      ->setConfigurationSourceList($config);
+    $loader->loadPackages();
+  }
+
   private function newToolset(array $argv) {
     $binary = basename($argv[0]);
 
diff --git a/src/workflow/ArcanistPackagesWorkflow.php b/src/workflow/ArcanistPackagesWorkflow.php
new file mode 100644
--- /dev/null
+++ b/src/workflow/ArcanistPackagesWorkflow.php
@@ -0,0 +1,193 @@
+<?php
+
+final class ArcanistPackagesWorkflow extends ArcanistWorkflow {
+
+  public function supportsToolset(ArcanistToolset $toolset) {
+    return true;
+  }
+
+  public function getWorkflowName() {
+    return 'packages';
+  }
+
+  public function getWorkflowInformation() {
+    $help = pht(<<<EOTEXT
+Supports: cli
+When used with no arguments, lists available and configured packages.
+
+**WARNING**: this workflow is highly unstable. Just ignore it for now.
+EOTEXT
+);
+    return $this->newWorkflowInformation()
+      ->setSynopsis(pht('Manage Extension Packages'))
+      ->addExample(pht('**packages**'))
+      ->addExample(pht('**packages** --install-from-tgz __file__'))
+      ->setHelp($help);
+  }
+
+  public function getWorkflowArguments() {
+    return array(
+      $this->newWorkflowArgument('install-from-tgz')
+        ->setParameter('package_archive')
+        ->setHelp(pht('UNSTABLE: Install package from an archive')),
+    );
+  }
+
+  public function runWorkflow() {
+    $fromtar = $this->getArgument('install-from-tgz');
+    if ($fromtar !== null) {
+      $this->runInstallFromTar($fromtar);
+      return;
+    }
+
+    $this->runListPackagesWorkflow();
+  }
+
+  private function runInstallFromTar($tar_file) {
+    Filesystem::assertReadable($tar_file);
+    Filesystem::assertIsFile($tar_file);
+
+    list($stdout, $stderr) = execx(
+        'tar -x -z -O -f %s ./.arcpackage',
+        $tar_file);
+    $manifest = phutil_json_decode($stdout);
+    $descriptor = ArcanistPackageDescriptor::fromManifest($manifest);
+
+    $dir = $this->resolvePath($descriptor);
+    if (Filesystem::pathExists($dir)) {
+      throw new ArcanistUsageException(
+        pht('this package already installed at %s', $dir));
+    }
+
+    Filesystem::createDirectory($dir, 0755, true);
+    execx('tar -x -z -f %s -C %s', $tar_file, $dir);
+  }
+
+  private function runListPackagesWorkflow() {
+    $console = PhutilConsole::getConsole();
+
+    $loader = id(new ArcanistPackagesLoader())
+      ->setConfigurationSourceList($this->getConfigurationSourceList());
+
+    $configured = $loader->listRequestedPackages();
+    $configured = mpull($configured, null, 'getDirectoryName');
+
+    $installed = $loader->listInstalledPackages();
+    $installed = mpull($installed, null, 'getDirectoryName');
+
+    $report = array();
+
+    foreach ($configured as $key => $descriptor) {
+      $report[$key] = array(
+        'descriptor' => $descriptor,
+        'status' => 'requested',
+      );
+    }
+
+    foreach ($installed as $key => $descriptor) {
+      if (array_key_exists($key, $report)) {
+        $report[$key]['status'] = 'loaded';
+        $report[$key]['descriptor']->setDescription(
+          $descriptor->getDescription());
+      } else {
+        $report[$key] = array(
+          'descriptor' => $descriptor,
+          'status' => 'installed',
+        );
+      }
+    }
+
+    $color_map = array(
+      'loaded' => 'green',
+      'installed' => 'yellow',
+      'requested' => 'red',
+    );
+    $text_map = $this->getPackageStatusMap();
+
+    ksort($report);
+
+    foreach ($report as $ident => $package) {
+      $status = $package['status'];
+      $color = $color_map[$status];
+      $text = $text_map[$status];
+
+      $descriptor = $package['descriptor'];
+
+      $description = $descriptor->getDescription();
+      if (!strlen($description)) {
+        $description = pht('Mystery package');
+      }
+      $config_text = null;
+      $config_source = $descriptor->getConfigSource();
+      if ($config_source) {
+        $config_text = pht(
+          '(Requested at %s)',
+          $config_source->getSourceDisplayName());
+      }
+      $console->writeOut(
+        "<bg:".$color.">** %s **</bg> **%s** %s (%s) %s\n",
+        $text,
+        $descriptor->getIdentifier(),
+        $descriptor->getVersion(),
+        $description,
+        $config_text);
+    }
+  }
+
+  private function loadConfiguredPackages() {
+    $items = $this->getConfigurationSourceList()
+      ->getStorageValueList('packages');
+
+    $packages = array();
+    foreach ($items as $item) {
+      foreach ($item->getValue() as $package_item) {
+        $packages[] = $package_item + array(
+          'config-source' => $item->getConfigurationSource(),
+        );
+      }
+    }
+    return $packages;
+  }
+
+   /**
+   * Get human-readable statuses, padded to fixed width.
+   *
+   * @return map<string, string> Human-readable status names.
+   */
+  // TODO this function is copied from Linters workflow
+  private function getPackageStatusMap() {
+    $text_map = array(
+      'loaded' => pht('LOADED'),
+      'installed' => pht('AVAILABLE'),
+      'requested' => pht('MISSING'),
+    );
+
+    $sizes = array();
+    foreach ($text_map as $key => $string) {
+      $sizes[$key] = phutil_utf8_console_strlen($string);
+    }
+
+    $longest = max($sizes);
+    foreach ($text_map as $key => $string) {
+      if ($sizes[$key] < $longest) {
+        $text_map[$key] .= str_repeat(' ', $longest - $sizes[$key]);
+      }
+    }
+
+    $text_map['padding'] = str_repeat(' ', $longest);
+
+    return $text_map;
+  }
+
+  // TODO these 2 are copy-pasted from ArcanistPackagesLoader
+  private function resolvePath(ArcanistPackageDescriptor $package) {
+    return $this->getPackagesDirectory().'/'.$package->getDirectoryName();
+  }
+
+  private function getPackagesDirectory() {
+    if (phutil_is_windows()) {
+      return getenv('APPDATA').'/.cache/arcanist-packages';
+    }
+    return getenv('HOME').'/.cache/arcanist-packages';
+  }
+}