diff --git a/tests/src/Unit/ConfigListerTest.php b/tests/src/Unit/ConfigListerTest.php
new file mode 100644
index 0000000..2eb7b59
--- /dev/null
+++ b/tests/src/Unit/ConfigListerTest.php
@@ -0,0 +1,226 @@
+<?php
+
+namespace Drupal\Tests\config_update\Unit;
+
+use Drupal\config_update\ConfigLister;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Tests the \Drupal\config_update\ConfigLister class.
+ *
+ * @group config_update
+ *
+ * @coversDefaultClass \Drupal\config_update\ConfigLister
+ */
+class ConfigListerTest extends UnitTestCase {
+
+  /**
+   * The config lister to test.
+   *
+   * @var \Drupal\config_update\ConfigLister
+   */
+  protected $configLister;
+
+  /**
+   * The mocked entity definition information.
+   *
+   * @var string[]
+   */
+  protected $entityDefinitionInformation;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    $this->configLister = new ConfigLister($this->getEntityManagerMock(), $this->getConfigStorageMock('active'), $this->getConfigStorageMock('extension'), $this->getConfigStorageMock('optional'));
+  }
+
+  /**
+   * Creates a mock entity manager for the test.
+   */
+  protected function getEntityManagerMock() {
+    // Make a list of fake entity definitions. Make sure they are not sorted,
+    // to test that the methods sort them. Also make sure there are a couple
+    // with prefixes that are subsets of each other.
+    $this->entityDefinitionInformation = [
+      ['prefix' => 'foo.bar', 'type' => 'foo'],
+      ['prefix' => 'foo.barbaz', 'type' => 'bar'],
+      ['prefix' => 'baz.foo', 'type' => 'baz'],
+    ];
+
+    $definitions = [];
+    foreach($this->entityDefinitionInformation as $info) {
+      $def = $this->getMockBuilder('Drupal\Core\Config\Entity\ConfigEntityTypeInterface')->getMock();
+      $def
+        ->expects($this->any())
+        ->method('getConfigPrefix')
+        ->willReturn($info['prefix']);
+      $def
+        ->expects($this->any())
+        ->method('isSubclassOf')
+        ->willReturn(TRUE);
+
+      $def->getConfigPrefix();
+
+      $definitions[$info['type']] = $def;
+    }
+
+    $manager = $this->getMockBuilder('Drupal\Core\Entity\EntityTypeManagerInterface')->getMock();
+    $manager
+      ->method('getDefinitions')
+      ->willReturn($definitions);
+
+    return($manager);
+  }
+
+  /**
+   * Creates a mock config storage object for the test.
+   *
+   * @param string $type
+   *   Type of storage object to return: 'active', 'extension', or 'optional'.
+   */
+  protected function getConfigStorageMock($type) {
+    if ($type == 'active') {
+      $storage = $this->getMockBuilder('Drupal\Core\Config\StorageInterface')->getMock();
+
+      // The only use of the read() method on active storage is
+      // with the core.extension config, to get the profile name.
+      $storage
+        ->method('read')
+        ->willReturn(['profile' => 'standard']);
+
+      $map = [
+        ['foo.bar', ['foo.bar.one', 'foo.bar.two', 'foo.bar.three']],
+        ['foo.barbaz', ['foo.barbaz.four', 'foo.barbaz.five', 'foo.barbaz.six']],
+        ['baz.foo'], [],
+        ['', ['foo.bar.one', 'foo.bar.two', 'foo.bar.three', 'foo.barbaz.four', 'foo.barbaz.five', 'foo.barbaz.six', 'something.else', 'another.one']],
+      ];
+
+      $storage
+        ->method('listAll')
+        ->will($this->returnValueMap($map));
+    }
+    elseif ($type == 'extension') {
+      $storage = $this->getMockBuilder('Drupal\Core\Config\ExtensionInstallStorage')->disableOriginalConstructor()->getMock();
+      $storage
+        ->method('getComponentNames')
+        ->willReturn([
+          'foo.bar.one' => 'ignored',
+          'foo.bar.two' => 'ignored',
+          'foo.bar.seven' => 'ignored',
+          'foo.barnot.three' => 'ignored',
+          'something.else' => 'ignored',
+          ]);
+
+      $map = [
+        ['foo.bar', ['foo.bar.one', 'foo.bar.two', 'foo.bar.seven']],
+        ['baz.foo'], [],
+        ['', ['foo.bar.one', 'foo.bar.two', 'foo.bar.seven', 'foo.barbaz.four', 'foo.barnot.three', 'something.else']],
+      ];
+
+      $storage
+        ->method('listAll')
+        ->will($this->returnValueMap($map));
+    }
+    else {
+      $storage = $this->getMockBuilder('Drupal\Core\Config\ExtensionInstallStorage')->disableOriginalConstructor()->getMock();
+      $storage
+        ->method('getComponentNames')
+        ->willReturn([
+          'foo.barbaz.four' => 'ignored',
+        ]);
+
+      $map = [
+        ['foo.bar'], [],
+        ['foo.barbaz', ['foo.barbaz.four']],
+        ['', ['foo.barbaz.four']],
+      ];
+      $storage
+        ->method('listAll')
+        ->will($this->returnValueMap($map));
+    }
+    return $storage;
+
+  }
+
+  /**
+   * @covers \Drupal\config_update\ConfigLister::listConfig
+   * @dataProvider listConfigProvider
+   */
+  public function testListConfig($a, $b, $expected) {
+    $this->assertEquals($expected, $this->configLister->listConfig($a, $b));
+  }
+
+  /**
+   * Data provider for self:testListConfig().
+   */
+  public function listConfigProvider() {
+    return [
+      // Arguments are $list_type, $name.
+      // We cannot really test the extension types here, because they rely
+      // on the going out to the file system to find out what config objects
+      // are there. This is too complex to mock. It is tested in the tests for
+      // the report output in the config_update_ui module tests. Anyway, we
+      // can test the other types.
+      ['type', 'system.all', [
+        ['foo.bar.one', 'foo.bar.two', 'foo.bar.three', 'foo.barbaz.four', 'foo.barbaz.five', 'foo.barbaz.six', 'something.else', 'another.one'],
+        ['foo.bar.one', 'foo.bar.two', 'foo.bar.seven', 'foo.barbaz.four', 'foo.barnot.three', 'something.else'],
+        ['foo.barbaz.four'],
+        ]
+      ],
+      ['type', 'system.simple', [
+        ['something.else', 'another.one'],
+        ['foo.barnot.three', 'something.else'],
+        [],
+        ]
+      ],
+      ['type', 'foo', [
+        ['foo.bar.one', 'foo.bar.two', 'foo.bar.three'],
+        ['foo.bar.one', 'foo.bar.two', 'foo.bar.seven'],
+        [],
+        ]
+      ],
+      ['type', 'unknown.type', [[],[],[]]],
+    ];
+  }
+
+  /**
+   * @covers \Drupal\config_update\ConfigLister::getType
+   */
+  public function testGetType() {
+    $return = $this->configLister->getType('not_in_list');
+    $this->assertNull($return);
+
+    foreach($this->entityDefinitionInformation as $info) {
+      $return = $this->configLister->getType($info['type']);
+      $this->assertEquals($return->getConfigPrefix(), $info['prefix']);
+    }
+  }
+
+  /**
+   * @covers \Drupal\config_update\ConfigLister::getTypeByPrefix
+   */
+  public function testGetTypeByPrefix() {
+    $return = $this->configLister->getTypeByPrefix('not_in_list');
+    $this->assertNull($return);
+
+    foreach($this->entityDefinitionInformation as $info) {
+      $return = $this->configLister->getTypeByPrefix($info['prefix']);
+      $this->assertEquals($return->getConfigPrefix(), $info['prefix']);
+    }
+  }
+
+  /**
+   * @covers \Drupal\config_update\ConfigLister::getTypeNameByConfigName
+   */
+  public function testGetTypeNameByConfigName() {
+    $return = $this->configLister->getTypeNameByConfigName('not_in_list');
+    $this->assertNull($return);
+
+    foreach($this->entityDefinitionInformation as $info) {
+      $return = $this->configLister->getTypeNameByConfigName($info['prefix'] . '.something');
+      $this->assertEquals($return, $info['type']);
+    }
+  }
+
+}