diff --git a/src/main/scala/plugin/Plugin.scala b/src/main/scala/plugin/Plugin.scala
index 3c9fe4a..e797699 100644
--- a/src/main/scala/plugin/Plugin.scala
+++ b/src/main/scala/plugin/Plugin.scala
@@ -2,6 +2,10 @@
 
 import util.Version
 
+/**
+ * Trait for define plugin interface.
+ * To provide plugin, put Plugin class which mixed in this trait into the package root.
+ */
 trait Plugin {
 
   val pluginId: String
@@ -9,8 +13,16 @@
   val description: String
   val versions: Seq[Version]
 
+  /**
+   * This method is invoked in initialization of plugin system.
+   * Register plugin functionality to PluginRegistry.
+   */
   def initialize(registry: PluginRegistry): Unit
 
+  /**
+   * This method is invoked in shutdown of plugin system.
+   * If the plugin has any resources, release them in this method.
+   */
   def shutdown(registry: PluginRegistry): Unit
 
 }
diff --git a/src/main/scala/plugin/PluginRegistory.scala b/src/main/scala/plugin/PluginRegistory.scala
index 186b38a..c924922 100644
--- a/src/main/scala/plugin/PluginRegistory.scala
+++ b/src/main/scala/plugin/PluginRegistory.scala
@@ -73,14 +73,23 @@
 
 }
 
+/**
+ * Provides entry point to PluginRegistry.
+ */
 object PluginRegistry {
 
   private val logger = LoggerFactory.getLogger(classOf[PluginRegistry])
 
   private val instance = new PluginRegistry()
 
+  /**
+   * Returns the PluginRegistry singleton instance.
+   */
   def apply(): PluginRegistry = instance
 
+  /**
+   * Initializes all installed plugins.
+   */
   def initialize(conn: java.sql.Connection): Unit = {
     val pluginDir = new File(PluginHome)
     if(pluginDir.exists && pluginDir.isDirectory){
diff --git a/src/main/scala/plugin/Results.scala b/src/main/scala/plugin/Results.scala
index c31c170..18fdb7f 100644
--- a/src/main/scala/plugin/Results.scala
+++ b/src/main/scala/plugin/Results.scala
@@ -2,6 +2,9 @@
 
 import play.twirl.api.Html
 
+/**
+ * Defines result case classes returned by plugin controller.
+ */
 object Results {
   case class Redirect(path: String)
   case class Fragment(html: Html)