diff --git a/docs/dtd/plugin/2.0.0-01.dtd b/docs/dtd/plugin/2.0.0-01.dtd
index 954a2c2219..62907f9481 100644
--- a/docs/dtd/plugin/2.0.0-01.dtd
+++ b/docs/dtd/plugin/2.0.0-01.dtd
@@ -23,7 +23,7 @@
 -->
 
 <!--- root element of the plugin descriptor -->
-<!ELEMENT plugin (scm-version|information|child-first-classloader|conditions|resources|dependencies|extension|extension-point|rest-resource|subscriber)*>
+<!ELEMENT plugin (scm-version|information|child-first-classloader|conditions|resources|dependencies|optional-dependencies|extension|extension-point|rest-resource|subscriber)*>
 
 <!--- major scm-manager version -->
 <!ELEMENT scm-version (#PCDATA)>
@@ -79,7 +79,10 @@
 <!--- contains plugin dependencies -->
 <!ELEMENT dependencies (dependency)*>
 
-<!--- sing plugin dependency -->
+<!--- contains optional plugin dependencies -->
+<!ELEMENT optional-dependencies (dependency)*>
+
+<!--- single plugin dependency -->
 <!ELEMENT dependency (#PCDATA)>
 
 <!-- generated entries -->
diff --git a/scm-annotations/src/main/java/sonia/scm/plugin/Extension.java b/scm-annotations/src/main/java/sonia/scm/plugin/Extension.java
index ce3bd24406..948dd46200 100644
--- a/scm-annotations/src/main/java/sonia/scm/plugin/Extension.java
+++ b/scm-annotations/src/main/java/sonia/scm/plugin/Extension.java
@@ -49,4 +49,15 @@ import java.lang.annotation.Target;
 @Target({ ElementType.TYPE })
 @PluginAnnotation("extension")
 @Retention(RetentionPolicy.RUNTIME)
-public @interface Extension {}
+public @interface Extension {
+  /**
+   * List of required plugins to load this extension.
+   * The requires attribute can be used to implement optional extensions.
+   * A plugin author is able to implement an extension point of an optional plugin and the extension is only loaded if
+   * all of the specified plugins are installed.
+   *
+   * @since 2.0.0
+   * @return list of required plugins to load this extension
+   */
+  String[] requires() default {};
+}
diff --git a/scm-core/src/main/java/sonia/scm/plugin/ExtensionElement.java b/scm-core/src/main/java/sonia/scm/plugin/ExtensionElement.java
new file mode 100644
index 0000000000..1dd6f51291
--- /dev/null
+++ b/scm-core/src/main/java/sonia/scm/plugin/ExtensionElement.java
@@ -0,0 +1,26 @@
+package sonia.scm.plugin;
+
+import lombok.AllArgsConstructor;
+import lombok.EqualsAndHashCode;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.ToString;
+
+import javax.xml.bind.annotation.XmlAccessType;
+import javax.xml.bind.annotation.XmlAccessorType;
+import javax.xml.bind.annotation.XmlElement;
+import java.util.HashSet;
+import java.util.Set;
+
+@Getter
+@ToString
+@NoArgsConstructor
+@EqualsAndHashCode
+@AllArgsConstructor
+@XmlAccessorType(XmlAccessType.FIELD)
+public class ExtensionElement {
+  @XmlElement(name = "class")
+  private String clazz;
+  private String description;
+  private Set<String> requires = new HashSet<>();
+}
diff --git a/scm-core/src/main/java/sonia/scm/plugin/InstalledPluginDescriptor.java b/scm-core/src/main/java/sonia/scm/plugin/InstalledPluginDescriptor.java
index 88ae4c5dff..097a06ab2c 100644
--- a/scm-core/src/main/java/sonia/scm/plugin/InstalledPluginDescriptor.java
+++ b/scm-core/src/main/java/sonia/scm/plugin/InstalledPluginDescriptor.java
@@ -38,6 +38,7 @@ package sonia.scm.plugin;
 import com.google.common.base.MoreObjects;
 import com.google.common.base.Objects;
 import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.Iterables;
 
 import javax.xml.bind.annotation.XmlAccessType;
 import javax.xml.bind.annotation.XmlAccessorType;
@@ -76,7 +77,7 @@ public final class InstalledPluginDescriptor extends ScmModule implements Plugin
    */
   public InstalledPluginDescriptor(int scmVersion, PluginInformation information,
                                    PluginResources resources, PluginCondition condition,
-                                   boolean childFirstClassLoader, Set<String> dependencies)
+                                   boolean childFirstClassLoader, Set<String> dependencies, Set<String> optionalDependencies)
   {
     this.scmVersion = scmVersion;
     this.information = information;
@@ -84,6 +85,7 @@ public final class InstalledPluginDescriptor extends ScmModule implements Plugin
     this.condition = condition;
     this.childFirstClassLoader = childFirstClassLoader;
     this.dependencies = dependencies;
+    this.optionalDependencies = optionalDependencies;
   }
 
   //~--- methods --------------------------------------------------------------
@@ -116,7 +118,8 @@ public final class InstalledPluginDescriptor extends ScmModule implements Plugin
       && Objects.equal(information, other.information)
       && Objects.equal(resources, other.resources)
       && Objects.equal(childFirstClassLoader, other.childFirstClassLoader)
-      && Objects.equal(dependencies, other.dependencies);
+      && Objects.equal(dependencies, other.dependencies)
+      && Objects.equal(optionalDependencies, other.optionalDependencies);
   }
 
   /**
@@ -129,7 +132,7 @@ public final class InstalledPluginDescriptor extends ScmModule implements Plugin
   public int hashCode()
   {
     return Objects.hashCode(scmVersion, condition, information, resources,
-      childFirstClassLoader, dependencies);
+      childFirstClassLoader, dependencies, optionalDependencies);
   }
 
   /**
@@ -149,6 +152,7 @@ public final class InstalledPluginDescriptor extends ScmModule implements Plugin
                   .add("resources", resources)
                   .add("childFirstClassLoader", childFirstClassLoader)
                   .add("dependencies", dependencies)
+                  .add("optionalDependencies", optionalDependencies)
                   .toString();
     //J+
   }
@@ -186,6 +190,27 @@ public final class InstalledPluginDescriptor extends ScmModule implements Plugin
     return dependencies;
   }
 
+  /**
+   * Method description
+   *
+   *
+   * @return
+   *
+   * @since 2.0.0
+   */
+  public Set<String> getOptionalDependencies() {
+    if (optionalDependencies == null)
+    {
+      optionalDependencies = ImmutableSet.of();
+    }
+
+    return optionalDependencies;
+  }
+
+  public Set<String> getDependenciesInclusiveOptionals() {
+    return ImmutableSet.copyOf(Iterables.concat(getDependencies(), getOptionalDependencies()));
+  }
+
   /**
    * Method description
    *
@@ -246,6 +271,11 @@ public final class InstalledPluginDescriptor extends ScmModule implements Plugin
   @XmlElementWrapper(name = "dependencies")
   private Set<String> dependencies;
 
+  /** Field description */
+  @XmlElement(name = "dependency")
+  @XmlElementWrapper(name = "optional-dependencies")
+  private Set<String> optionalDependencies;
+
   /** Field description */
   @XmlElement(name = "information")
   private PluginInformation information;
diff --git a/scm-core/src/main/java/sonia/scm/plugin/ScmModule.java b/scm-core/src/main/java/sonia/scm/plugin/ScmModule.java
index 683b2a7e0c..398698273c 100644
--- a/scm-core/src/main/java/sonia/scm/plugin/ScmModule.java
+++ b/scm-core/src/main/java/sonia/scm/plugin/ScmModule.java
@@ -89,9 +89,9 @@ public class ScmModule
    *
    * @return
    */
-  public Iterable<Class<?>> getExtensions()
+  public Iterable<ExtensionElement> getExtensions()
   {
-    return unwrap(extensions);
+    return nonNull(extensions);
   }
 
   /**
@@ -223,7 +223,7 @@ public class ScmModule
 
   /** Field description */
   @XmlElement(name = "extension")
-  private Set<ClassElement> extensions;
+  private Set<ExtensionElement> extensions;
 
   /** Field description */
   @XmlElement(name = "rest-provider")
diff --git a/scm-core/src/test/java/sonia/scm/plugin/ScmModuleTest.java b/scm-core/src/test/java/sonia/scm/plugin/ScmModuleTest.java
index 1629562132..86bd7fe19d 100644
--- a/scm-core/src/test/java/sonia/scm/plugin/ScmModuleTest.java
+++ b/scm-core/src/test/java/sonia/scm/plugin/ScmModuleTest.java
@@ -33,6 +33,7 @@ package sonia.scm.plugin;
 
 //~--- non-JDK imports --------------------------------------------------------
 
+import com.google.common.collect.Iterables;
 import com.google.common.io.Resources;
 
 import org.junit.Test;
@@ -69,43 +70,43 @@ public class ScmModuleTest
 
     //J-
     assertThat(
-      module.getExtensions(), 
+      Iterables.transform(module.getExtensions(), ExtensionElement::getClazz),
       containsInAnyOrder(
-        String.class,
-        Integer.class
+        String.class.getName(),
+        Integer.class.getName()
       )
     );
     assertThat(
-      module.getExtensionPoints(), 
+      module.getExtensionPoints(),
       containsInAnyOrder(
-        new ExtensionPointElement(String.class, "ext01", true, true), 
-        new ExtensionPointElement(Long.class, "ext02", true, true), 
+        new ExtensionPointElement(String.class, "ext01", true, true),
+        new ExtensionPointElement(Long.class, "ext02", true, true),
         new ExtensionPointElement(Integer.class, "ext03", false, true)
       )
     );
     assertThat(
-      module.getEvents(), 
+      module.getEvents(),
       containsInAnyOrder(
         String.class,
         Boolean.class
       )
     );
     assertThat(
-      module.getSubscribers(), 
+      module.getSubscribers(),
       containsInAnyOrder(
-        new SubscriberElement(Long.class, Integer.class, "sub01"), 
+        new SubscriberElement(Long.class, Integer.class, "sub01"),
         new SubscriberElement(Double.class, Float.class, "sub02")
       )
     );
     assertThat(
-      module.getRestProviders(), 
+      module.getRestProviders(),
       containsInAnyOrder(
         Integer.class,
         Long.class
       )
     );
     assertThat(
-      module.getRestResources(), 
+      module.getRestResources(),
       containsInAnyOrder(
         Float.class,
         Double.class
diff --git a/scm-core/src/test/resources/sonia/scm/plugin/module.xml b/scm-core/src/test/resources/sonia/scm/plugin/module.xml
index c135803a7d..0bea44d06e 100644
--- a/scm-core/src/test/resources/sonia/scm/plugin/module.xml
+++ b/scm-core/src/test/resources/sonia/scm/plugin/module.xml
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
 <module>
-  
+
   <!-- subscribers -->
   <subscriber>
     <class>java.lang.Long</class>
@@ -47,28 +47,32 @@
 
   <extension>
     <class>java.lang.String</class>
+    <requires>scm-mail-plugin</requires>
+    <requires>scm-review-plugin</requires>
+    <description>ext01</description>
   </extension>
 
   <extension>
     <class>java.lang.Integer</class>
+    <description>ext02</description>
   </extension>
-  
+
   <!-- rest providers -->
-  
+
   <rest-provider>
     <class>java.lang.Integer</class>
   </rest-provider>
-  
+
   <rest-provider>
     <class>java.lang.Long</class>
   </rest-provider>
-  
+
   <!-- jax-rs resources -->
-  
+
   <rest-resource>
     <class>java.lang.Float</class>
   </rest-resource>
-  
+
   <rest-resource>
     <class>java.lang.Double</class>
   </rest-resource>
diff --git a/scm-webapp/src/main/java/sonia/scm/plugin/DefaultPluginLoader.java b/scm-webapp/src/main/java/sonia/scm/plugin/DefaultPluginLoader.java
index 5612b0395c..e5ab89821f 100644
--- a/scm-webapp/src/main/java/sonia/scm/plugin/DefaultPluginLoader.java
+++ b/scm-webapp/src/main/java/sonia/scm/plugin/DefaultPluginLoader.java
@@ -99,7 +99,7 @@ public class DefaultPluginLoader implements PluginLoader
 
       modules = getInstalled(parent, context, PATH_MODULECONFIG);
 
-      collector = new ExtensionCollector(Iterables.concat(modules, unwrap()));
+      collector = new ExtensionCollector(parent, modules, installedPlugins);
       extensionProcessor = new DefaultExtensionProcessor(collector);
     }
     catch (IOException | JAXBException ex)
@@ -170,19 +170,6 @@ public class DefaultPluginLoader implements PluginLoader
     return uberWebResourceLoader;
   }
 
-  //~--- methods --------------------------------------------------------------
-
-  /**
-   * Method description
-   *
-   *
-   * @return
-   */
-  private Iterable<InstalledPluginDescriptor> unwrap()
-  {
-    return PluginsInternal.unwrap(installedPlugins);
-  }
-
   //~--- get methods ----------------------------------------------------------
 
   /**
diff --git a/scm-webapp/src/main/java/sonia/scm/plugin/ExplodedSmp.java b/scm-webapp/src/main/java/sonia/scm/plugin/ExplodedSmp.java
index ee2514dc3e..60bcc09dcb 100644
--- a/scm-webapp/src/main/java/sonia/scm/plugin/ExplodedSmp.java
+++ b/scm-webapp/src/main/java/sonia/scm/plugin/ExplodedSmp.java
@@ -98,8 +98,8 @@ public final class ExplodedSmp implements Comparable<ExplodedSmp>
   {
     int result;
 
-    Set<String> depends = plugin.getDependencies();
-    Set<String> odepends = o.plugin.getDependencies();
+    Set<String> depends = plugin.getDependenciesInclusiveOptionals();
+    Set<String> odepends = o.plugin.getDependenciesInclusiveOptionals();
 
     if (depends.isEmpty() && odepends.isEmpty())
     {
diff --git a/scm-webapp/src/main/java/sonia/scm/plugin/ExtensionCollector.java b/scm-webapp/src/main/java/sonia/scm/plugin/ExtensionCollector.java
index 9ecf9a9da8..8ddf847935 100644
--- a/scm-webapp/src/main/java/sonia/scm/plugin/ExtensionCollector.java
+++ b/scm-webapp/src/main/java/sonia/scm/plugin/ExtensionCollector.java
@@ -39,6 +39,8 @@ import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
 import com.google.common.collect.Multimap;
 import com.google.common.collect.Sets;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 //~--- JDK imports ------------------------------------------------------------
 
@@ -47,6 +49,7 @@ import java.util.Collections;
 import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Set;
+import java.util.stream.Collectors;
 
 /**
  *
@@ -56,23 +59,33 @@ import java.util.Set;
 public final class ExtensionCollector
 {
 
-  /**
-   * Constructs ...
-   *
-   *
-   * @param modules
-   */
-  ExtensionCollector(Iterable<ScmModule> modules)
-  {
-    for (ScmModule module : modules)
-    {
+  private static final Logger LOG = LoggerFactory.getLogger(ExtensionCollector.class);
+
+  private final Set<String> pluginIndex;
+
+  public ExtensionCollector(ClassLoader moduleClassLoader, Set<ScmModule> modules, Set<InstalledPlugin> installedPlugins) {
+    this.pluginIndex = createPluginIndex(installedPlugins);
+
+    for (ScmModule module : modules) {
       collectRootElements(module);
     }
-
-    for (ScmModule module : modules)
-    {
-      collectExtensions(module);
+    for (ScmModule plugin : PluginsInternal.unwrap(installedPlugins)) {
+      collectRootElements(plugin);
     }
+
+    for (ScmModule module : modules) {
+      collectExtensions(moduleClassLoader, module);
+    }
+
+    for (InstalledPlugin plugin : installedPlugins) {
+      collectExtensions(plugin.getClassLoader(), plugin.getDescriptor());
+    }
+  }
+
+  private Set<String> createPluginIndex(Set<InstalledPlugin> installedPlugins) {
+    return installedPlugins.stream()
+      .map(p -> p.getDescriptor().getInformation().getName())
+      .collect(Collectors.toSet());
   }
 
   //~--- methods --------------------------------------------------------------
@@ -245,20 +258,35 @@ public final class ExtensionCollector
     }
   }
 
-  /**
-   * Method description
-   *
-   *
-   * @param module
-   */
-  private void collectExtensions(ScmModule module)
-  {
-    for (Class extension : module.getExtensions())
-    {
-      appendExtension(extension);
+  private void collectExtensions(ClassLoader defaultClassLoader, ScmModule module) {
+    for (ExtensionElement extension : module.getExtensions()) {
+      if (isRequirementFulfilled(extension)) {
+        Class<?> extensionClass = loadExtension(defaultClassLoader, extension);
+        appendExtension(extensionClass);
+      }
     }
   }
 
+  private Class<?> loadExtension(ClassLoader classLoader, ExtensionElement extension) {
+    try {
+      return classLoader.loadClass(extension.getClazz());
+    } catch (ClassNotFoundException ex) {
+      throw new RuntimeException("failed to load clazz", ex);
+    }
+  }
+
+  private boolean isRequirementFulfilled(ExtensionElement extension) {
+    if (extension.getRequires() != null) {
+      for (String requiredPlugin : extension.getRequires()) {
+        if (!pluginIndex.contains(requiredPlugin)) {
+          LOG.debug("skip loading of extension {}, because the required plugin {} is not installed", extension.getClazz(), requiredPlugin);
+          return false;
+        }
+      }
+    }
+    return true;
+  }
+
   /**
    * Method description
    *
diff --git a/scm-webapp/src/main/java/sonia/scm/plugin/PluginProcessor.java b/scm-webapp/src/main/java/sonia/scm/plugin/PluginProcessor.java
index 2bbab3c650..431cb44e1b 100644
--- a/scm-webapp/src/main/java/sonia/scm/plugin/PluginProcessor.java
+++ b/scm-webapp/src/main/java/sonia/scm/plugin/PluginProcessor.java
@@ -189,7 +189,7 @@ public final class PluginProcessor
 
     PluginTree pluginTree = new PluginTree(smps);
 
-    logger.trace("build plugin tree: {}", pluginTree);
+    logger.info("install plugin tree:\n{}", pluginTree);
 
     List<PluginNode> rootNodes = pluginTree.getRootNodes();
 
diff --git a/scm-webapp/src/main/java/sonia/scm/plugin/PluginTree.java b/scm-webapp/src/main/java/sonia/scm/plugin/PluginTree.java
index bd338c0741..2ebe06db02 100644
--- a/scm-webapp/src/main/java/sonia/scm/plugin/PluginTree.java
+++ b/scm-webapp/src/main/java/sonia/scm/plugin/PluginTree.java
@@ -35,15 +35,13 @@ package sonia.scm.plugin;
 
 import com.google.common.collect.Lists;
 import com.google.common.collect.Ordering;
-
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-//~--- JDK imports ------------------------------------------------------------
-
 import java.util.Arrays;
 import java.util.List;
-import java.util.Set;
+
+//~--- JDK imports ------------------------------------------------------------
 
 /**
  *
@@ -104,9 +102,7 @@ public final class PluginTree
 
       if ((condition == null) || condition.isSupported())
       {
-        Set<String> dependencies = plugin.getDependencies();
-
-        if ((dependencies == null) || dependencies.isEmpty())
+        if (plugin.getDependencies().isEmpty() && plugin.getOptionalDependencies().isEmpty())
         {
           rootNodes.add(new PluginNode(smp));
         }
@@ -170,6 +166,20 @@ public final class PluginTree
         //J+
       }
     }
+
+    boolean rootNode = smp.getPlugin().getDependencies().isEmpty();
+    for (String dependency : smp.getPlugin().getOptionalDependencies()) {
+      if (appendNode(rootNodes, child, dependency)) {
+        rootNode = false;
+      } else {
+        logger.info("optional dependency {} of {} is not installed", dependency, child.getId());
+      }
+    }
+
+    if (rootNode) {
+      logger.info("could not find optional dependencies of {}, append it as root node", child.getId());
+      rootNodes.add(new PluginNode(smp));
+    }
   }
 
   /**
@@ -212,7 +222,18 @@ public final class PluginTree
 
   @Override
   public String toString() {
-    return "plugin tree: " + rootNodes.toString();
+    StringBuilder buffer = new StringBuilder();
+    for (PluginNode node : rootNodes) {
+      append(buffer, "", node);
+    }
+    return buffer.toString();
+  }
+
+  private void append(StringBuilder buffer, String indent, PluginNode node) {
+    buffer.append(indent).append("+- ").append(node.getId()).append("\n");
+    for (PluginNode child : node.getChildren()) {
+      append(buffer, indent + "   ", child);
+    }
   }
 //~--- fields ---------------------------------------------------------------
 
diff --git a/scm-webapp/src/test/java/sonia/scm/plugin/ExplodedSmpTest.java b/scm-webapp/src/test/java/sonia/scm/plugin/ExplodedSmpTest.java
index 090d903f49..c4a4c0a0d3 100644
--- a/scm-webapp/src/test/java/sonia/scm/plugin/ExplodedSmpTest.java
+++ b/scm-webapp/src/test/java/sonia/scm/plugin/ExplodedSmpTest.java
@@ -134,7 +134,7 @@ public class ExplodedSmpTest
     info.setVersion(version);
 
     InstalledPluginDescriptor plugin = new InstalledPluginDescriptor(2, info, null, null, false,
-                      Sets.newSet(dependencies));
+                      Sets.newSet(dependencies), null);
 
     return new ExplodedSmp(null, plugin);
   }
diff --git a/scm-webapp/src/test/java/sonia/scm/plugin/PluginTreeTest.java b/scm-webapp/src/test/java/sonia/scm/plugin/PluginTreeTest.java
index 72c48d4d16..33a139aa5a 100644
--- a/scm-webapp/src/test/java/sonia/scm/plugin/PluginTreeTest.java
+++ b/scm-webapp/src/test/java/sonia/scm/plugin/PluginTreeTest.java
@@ -34,6 +34,7 @@ package sonia.scm.plugin;
 //~--- non-JDK imports --------------------------------------------------------
 
 import com.google.common.base.Function;
+import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Lists;
 
 import org.junit.Rule;
@@ -72,7 +73,7 @@ public class PluginTreeTest
     PluginCondition condition = new PluginCondition("999",
                                   new ArrayList<String>(), "hit");
     InstalledPluginDescriptor plugin = new InstalledPluginDescriptor(2, createInfo("a",  "1"), null, condition,
-                      false, null);
+                      false, null, null);
     ExplodedSmp smp = createSmp(plugin);
 
     new PluginTree(smp).getRootNodes();
@@ -115,7 +116,7 @@ public class PluginTreeTest
   public void testScmVersion() throws IOException
   {
     InstalledPluginDescriptor plugin = new InstalledPluginDescriptor(1, createInfo("a", "1"), null, null, false,
-                      null);
+                      null, null);
     ExplodedSmp smp = createSmp(plugin);
 
     new PluginTree(smp).getRootNodes();
@@ -131,10 +132,10 @@ public class PluginTreeTest
   public void testSimpleDependencies() throws IOException
   {
     //J-
-    ExplodedSmp[] smps = new ExplodedSmp[] { 
+    ExplodedSmp[] smps = new ExplodedSmp[] {
       createSmpWithDependency("a"),
       createSmpWithDependency("b", "a"),
-      createSmpWithDependency("c", "a", "b") 
+      createSmpWithDependency("c", "a", "b")
     };
     //J+
 
@@ -152,6 +153,61 @@ public class PluginTreeTest
     assertThat(unwrapIds(b.getChildren()), containsInAnyOrder("c"));
   }
 
+  @Test
+  public void testWithOptionalDependency() throws IOException {
+    ExplodedSmp[] smps = new ExplodedSmp[] {
+      createSmpWithDependency("a"),
+      createSmpWithDependency("b", null, ImmutableSet.of("a")),
+      createSmpWithDependency("c", null, ImmutableSet.of("a", "b"))
+    };
+
+    PluginTree tree = new PluginTree(smps);
+    List<PluginNode> rootNodes = tree.getRootNodes();
+
+    assertThat(unwrapIds(rootNodes), containsInAnyOrder("a"));
+
+    PluginNode a = rootNodes.get(0);
+    assertThat(unwrapIds(a.getChildren()), containsInAnyOrder("b", "c"));
+
+    PluginNode b = a.getChild("b");
+    assertThat(unwrapIds(b.getChildren()), containsInAnyOrder("c"));
+  }
+
+  @Test
+  public void testWithDeepOptionalDependency() throws IOException {
+    ExplodedSmp[] smps = new ExplodedSmp[] {
+      createSmpWithDependency("a"),
+      createSmpWithDependency("b", "a"),
+      createSmpWithDependency("c", null, ImmutableSet.of("b"))
+    };
+
+    PluginTree tree = new PluginTree(smps);
+
+    System.out.println(tree);
+
+    List<PluginNode> rootNodes = tree.getRootNodes();
+
+    assertThat(unwrapIds(rootNodes), containsInAnyOrder("a"));
+
+    PluginNode a = rootNodes.get(0);
+    assertThat(unwrapIds(a.getChildren()), containsInAnyOrder("b"));
+
+    PluginNode b = a.getChild("b");
+    assertThat(unwrapIds(b.getChildren()), containsInAnyOrder("c"));
+  }
+
+  @Test
+  public void testWithNonExistentOptionalDependency() throws IOException {
+    ExplodedSmp[] smps = new ExplodedSmp[] {
+      createSmpWithDependency("a", null, ImmutableSet.of("b"))
+    };
+
+    PluginTree tree = new PluginTree(smps);
+    List<PluginNode> rootNodes = tree.getRootNodes();
+
+    assertThat(unwrapIds(rootNodes), containsInAnyOrder("a"));
+  }
+
   /**
    * Method description
    *
@@ -200,7 +256,7 @@ public class PluginTreeTest
   private ExplodedSmp createSmp(String name) throws IOException
   {
     return createSmp(new InstalledPluginDescriptor(2, createInfo(name, "1.0.0"), null, null,
-      false, null));
+      false, null, null));
   }
 
   /**
@@ -224,10 +280,19 @@ public class PluginTreeTest
     {
       dependencySet.add(d);
     }
+    return createSmpWithDependency(name, dependencySet, null);
+  }
 
-    InstalledPluginDescriptor plugin = new InstalledPluginDescriptor(2, createInfo(name, "1"), null, null,
-                      false, dependencySet);
-
+  private ExplodedSmp createSmpWithDependency(String name, Set<String> dependencies, Set<String> optionalDependencies) throws IOException {
+    InstalledPluginDescriptor plugin = new InstalledPluginDescriptor(
+      2,
+      createInfo(name, "1"),
+      null,
+      null,
+      false,
+      dependencies,
+      optionalDependencies
+    );
     return createSmp(plugin);
   }