diff --git a/api/src/main/java/com/velocitypowered/api/proxy/ServerConnection.java b/api/src/main/java/com/velocitypowered/api/proxy/ServerConnection.java
index 080930c89..5fb44a44a 100644
--- a/api/src/main/java/com/velocitypowered/api/proxy/ServerConnection.java
+++ b/api/src/main/java/com/velocitypowered/api/proxy/ServerConnection.java
@@ -8,7 +8,15 @@ import com.velocitypowered.api.server.ServerInfo;
  * Represents a connection to a backend server from the proxy for a client.
  */
 public interface ServerConnection extends ChannelMessageSource, ChannelMessageSink {
+    /**
+     * Returns the server that this connection is connected to.
+     * @return the server this connection is connected to
+     */
     ServerInfo getServerInfo();
 
+    /**
+     * Returns the player that this connection is associated with.
+     * @return the player for this connection
+     */
     Player getPlayer();
 }
diff --git a/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelMessageSink.java b/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelMessageSink.java
index d813c4edc..57ebe4ea6 100644
--- a/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelMessageSink.java
+++ b/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelMessageSink.java
@@ -1,5 +1,13 @@
 package com.velocitypowered.api.proxy.messages;
 
+/**
+ * Represents something that can send plugin messages.
+ */
 public interface ChannelMessageSink {
+    /**
+     * Sends a plugin message to this target.
+     * @param identifier the channel identifier to send the message on
+     * @param data the data to send
+     */
     void sendPluginMessage(ChannelIdentifier identifier, byte[] data);
 }
diff --git a/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelMessageSource.java b/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelMessageSource.java
index 1aaeae943..8cb85ef1c 100644
--- a/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelMessageSource.java
+++ b/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelMessageSource.java
@@ -1,4 +1,7 @@
 package com.velocitypowered.api.proxy.messages;
 
+/**
+ * A marker interface that indicates a source of plugin messages.
+ */
 public interface ChannelMessageSource {
 }
diff --git a/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelRegistrar.java b/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelRegistrar.java
index 412e172df..2d77988b5 100644
--- a/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelRegistrar.java
+++ b/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelRegistrar.java
@@ -5,7 +5,16 @@ package com.velocitypowered.api.proxy.messages;
  * the client or the server.
  */
 public interface ChannelRegistrar {
+    /**
+     * Registers the specified message handler to listen for plugin messages on the specified channels.
+     * @param handler the handler to register
+     * @param identifiers the channel identifiers to register
+     */
     void register(MessageHandler handler, ChannelIdentifier... identifiers);
 
+    /**
+     * Unregisters the handler for the specified channel.
+     * @param identifiers the identifiers to unregister
+     */
     void unregister(ChannelIdentifier... identifiers);
 }
diff --git a/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelSide.java b/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelSide.java
index f27faf71b..12256f432 100644
--- a/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelSide.java
+++ b/api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelSide.java
@@ -1,6 +1,15 @@
 package com.velocitypowered.api.proxy.messages;
 
+/**
+ * Represents from "which side" of the proxy the plugin message came from.
+ */
 public enum ChannelSide {
+    /**
+     * The plugin message came from a server that a client was connected to.
+     */
     FROM_SERVER,
+    /**
+     * The plugin message came from the client.
+     */
     FROM_CLIENT
 }
diff --git a/api/src/main/java/com/velocitypowered/api/proxy/messages/MessageHandler.java b/api/src/main/java/com/velocitypowered/api/proxy/messages/MessageHandler.java
index 52f4b4e1a..70a7e5fa7 100644
--- a/api/src/main/java/com/velocitypowered/api/proxy/messages/MessageHandler.java
+++ b/api/src/main/java/com/velocitypowered/api/proxy/messages/MessageHandler.java
@@ -1,10 +1,28 @@
 package com.velocitypowered.api.proxy.messages;
 
+/**
+ * Represents a handler for handling plugin messages.
+ */
 public interface MessageHandler {
+    /**
+     * Handles an incoming plugin message.
+     * @param source the source of the plugin message
+     * @param side from where the plugin message originated
+     * @param identifier the channel on which the message was sent
+     * @param data the data inside the plugin message
+     * @return a {@link ForwardStatus} indicating whether or not to forward this plugin message on
+     */
     ForwardStatus handle(ChannelMessageSource source, ChannelSide side, ChannelIdentifier identifier, byte[] data);
 
     enum ForwardStatus {
+        /**
+         * Forwards this plugin message on to the client or server, depending on the {@link ChannelSide} it originated
+         * from.
+         */
         FORWARD,
+        /**
+         * Discard the plugin message and do not forward it on.
+         */
         HANDLED
     }
 }
diff --git a/api/src/main/java/com/velocitypowered/api/proxy/messages/MinecraftChannelIdentifier.java b/api/src/main/java/com/velocitypowered/api/proxy/messages/MinecraftChannelIdentifier.java
index c171fee66..1355ed239 100644
--- a/api/src/main/java/com/velocitypowered/api/proxy/messages/MinecraftChannelIdentifier.java
+++ b/api/src/main/java/com/velocitypowered/api/proxy/messages/MinecraftChannelIdentifier.java
@@ -20,10 +20,22 @@ public final class MinecraftChannelIdentifier implements ChannelIdentifier {
         this.name = name;
     }
 
+    /**
+     * Creates an identifier in the default namespace ({@code minecraft}). Plugins are strongly encouraged to provide
+     * their own namespace.
+     * @param name the name in the default namespace to use
+     * @return a new channel identifier
+     */
     public static MinecraftChannelIdentifier forDefaultNamespace(String name) {
         return new MinecraftChannelIdentifier("minecraft", name);
     }
 
+    /**
+     * Creates an identifier in the specified namespace.
+     * @param namespace the namespace to use
+     * @param name the channel name inside the specified namespace
+     * @return a new channel identifier
+     */
     public static MinecraftChannelIdentifier create(String namespace, String name) {
         Preconditions.checkArgument(!Strings.isNullOrEmpty(namespace), "namespace is null or empty");
         Preconditions.checkArgument(!Strings.isNullOrEmpty(name), "namespace is null or empty");
diff --git a/api/src/main/java/com/velocitypowered/api/scheduler/ScheduledTask.java b/api/src/main/java/com/velocitypowered/api/scheduler/ScheduledTask.java
index 58596c684..2ee41380f 100644
--- a/api/src/main/java/com/velocitypowered/api/scheduler/ScheduledTask.java
+++ b/api/src/main/java/com/velocitypowered/api/scheduler/ScheduledTask.java
@@ -4,9 +4,21 @@ package com.velocitypowered.api.scheduler;
  * Represents a task that is scheduled to run on the proxy.
  */
 public interface ScheduledTask {
+    /**
+     * Returns the plugin that scheduled this task.
+     * @return the plugin that scheduled this task
+     */
     Object plugin();
 
+    /**
+     * Returns the current status of this task.
+     * @return the current status of this task
+     */
     TaskStatus status();
 
+    /**
+     * Cancels this task. If the task is already running, the thread in which it is running will be interrupted.
+     * If the task is not currently running, Velocity will terminate it safely.
+     */
     void cancel();
 }
diff --git a/api/src/main/java/com/velocitypowered/api/scheduler/TaskStatus.java b/api/src/main/java/com/velocitypowered/api/scheduler/TaskStatus.java
index b5830fa51..e86f56e12 100644
--- a/api/src/main/java/com/velocitypowered/api/scheduler/TaskStatus.java
+++ b/api/src/main/java/com/velocitypowered/api/scheduler/TaskStatus.java
@@ -1,7 +1,16 @@
 package com.velocitypowered.api.scheduler;
 
 public enum TaskStatus {
+    /**
+     * The task is scheduled and is currently running.
+     */
     SCHEDULED,
+    /**
+     * The task was cancelled with {@link ScheduledTask#cancel()}.
+     */
     CANCELLED,
+    /**
+     * The task has run to completion. This is applicable only for tasks without a repeat.
+     */
     FINISHED
 }
diff --git a/api/src/main/java/com/velocitypowered/api/util/GameProfile.java b/api/src/main/java/com/velocitypowered/api/util/GameProfile.java
index f77b991f3..237939ef1 100644
--- a/api/src/main/java/com/velocitypowered/api/util/GameProfile.java
+++ b/api/src/main/java/com/velocitypowered/api/util/GameProfile.java
@@ -7,14 +7,17 @@ import org.checkerframework.checker.nullness.qual.NonNull;
 import java.util.List;
 import java.util.UUID;
 
+/**
+ * Represents a Mojang game profile.
+ */
 public class GameProfile {
     private final String id;
     private final String name;
     private final List<Property> properties;
 
     public GameProfile(@NonNull String id, @NonNull String name, @NonNull List<Property> properties) {
-        this.id = id;
-        this.name = name;
+        this.id = Preconditions.checkNotNull(id, "id");
+        this.name = Preconditions.checkNotNull(name, "name");
         this.properties = ImmutableList.copyOf(properties);
     }
 
@@ -31,7 +34,7 @@ public class GameProfile {
     }
 
     public List<Property> getProperties() {
-        return ImmutableList.copyOf(properties);
+        return properties;
     }
 
     public static GameProfile forOfflinePlayer(@NonNull String username) {
@@ -55,9 +58,9 @@ public class GameProfile {
         private final String signature;
 
         public Property(@NonNull String name, @NonNull String value, @NonNull String signature) {
-            this.name = name;
-            this.value = value;
-            this.signature = signature;
+            this.name = Preconditions.checkNotNull(name, "name");
+            this.value = Preconditions.checkNotNull(value, "value");
+            this.signature = Preconditions.checkNotNull(signature, "signature");
         }
 
         public String getName() {