diff --git a/package.yaml b/package.yaml
index 89d5327..ae735b3 100644
--- a/package.yaml
+++ b/package.yaml
@@ -14,6 +14,9 @@ extra-source-files:
 
 dependencies:
 - base >= 4.7 && < 5
+- aeson
+- text
+- unordered-containers
 
 library:
   source-dirs: src
diff --git a/src/Forest/Api.hs b/src/Forest/Api.hs
new file mode 100644
index 0000000..d9dfb31
--- /dev/null
+++ b/src/Forest/Api.hs
@@ -0,0 +1,143 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+-- | This module contains all the types found in the API.
+
+module Forest.Api
+  (
+  -- * Common
+    NodeId
+  , Node(..)
+  , Path(..)
+  -- * Client
+  , ClientPacket(..)
+  -- * Server
+  , ServerPacket(..)
+  ) where
+
+import           Control.Applicative
+import           Control.Monad
+import           Data.Aeson
+import           Data.Aeson.Types
+import qualified Data.HashMap.Strict as Map
+import qualified Data.Text           as T
+
+{- Common -}
+
+type NodeId = T.Text
+
+data Node = Node
+  { nodeText     :: T.Text
+  , nodeAct      :: Bool
+  , nodeEdit     :: Bool
+  , nodeDelete   :: Bool
+  , nodeReply    :: Bool
+  , nodeChildren :: Map.HashMap NodeId Node
+  } deriving (Show)
+
+instance ToJSON Node where
+  toJSON node = object
+    [ "text"     .= nodeText node
+    , "act"      .= nodeAct node
+    , "edit"     .= nodeEdit node
+    , "delete"   .= nodeDelete node
+    , "reply"    .= nodeReply node
+    , "children" .= nodeChildren node
+    ]
+
+  toEncoding node = pairs
+    (  "text"     .= nodeText node
+    <> "act"      .= nodeAct node
+    <> "edit"     .= nodeEdit node
+    <> "delete"   .= nodeDelete node
+    <> "reply"    .= nodeReply node
+    <> "children" .= nodeChildren node
+    )
+
+instance FromJSON Node where
+  parseJSON v = parseJSON v >>= \o -> Node
+    <$> o .: "text"
+    <*> o .: "act"
+    <*> o .: "edit"
+    <*> o .: "delete"
+    <*> o .: "reply"
+    <*> o .: "children"
+
+newtype Path = Path
+  { pathElements :: [NodeId]
+  } deriving (Show, Eq)
+
+instance ToJSON Path where
+  toJSON = toJSON . pathElements
+  toEncoding = toEncoding . pathElements
+
+instance FromJSON Path where
+  parseJSON v = Path <$> parseJSON v
+
+parsePacket :: Value -> T.Text -> (Object -> Parser a) -> Parser a
+parsePacket value packetType parser = parseJSON value >>= \o -> do
+  parsedPacketType <- o .: "type"
+  guard $ parsedPacketType == packetType
+  parser o
+
+{- Client -}
+
+data ClientPacket
+  = ClientHello [T.Text]
+  | ClientAct Path
+  | ClientEdit Path T.Text
+  | ClientDelete Path
+  | ClientReply Path T.Text
+  deriving (Show)
+
+instance ToJSON ClientPacket where
+  toJSON (ClientHello extensions) =
+    object ["type" .= ("hello" :: T.Text), "extensions" .= extensions]
+  toJSON (ClientAct path) = object ["type" .= ("act" :: T.Text), "path" .= path]
+  toJSON (ClientEdit path text) =
+    object ["type" .= ("edit" :: T.Text), "path" .= path, "text" .= text]
+  toJSON (ClientDelete path) =
+    object ["type" .= ("delete" :: T.Text), "path" .= path]
+  toJSON (ClientReply path text) =
+    object ["type" .= ("reply" :: T.Text), "path" .= path, "text" .= text]
+
+  toEncoding (ClientHello extensions) =
+    pairs ("type" .= ("hello" :: T.Text) <> "extensions" .= extensions)
+  toEncoding (ClientAct path) =
+    pairs ("type" .= ("act" :: T.Text) <> "path" .= path)
+  toEncoding (ClientEdit path text) =
+    pairs ("type" .= ("edit" :: T.Text) <> "path" .= path <> "text" .= text)
+  toEncoding (ClientDelete path) =
+    pairs ("type" .= ("delete" :: T.Text) <> "path" .= path)
+  toEncoding (ClientReply path text) =
+    pairs ("type" .= ("reply" :: T.Text) <> "path" .= path <> "text" .= text)
+
+instance FromJSON ClientPacket where
+  parseJSON v =
+    parsePacket v "hello"  (\o -> ClientHello  <$> o .: "extensions")           <|>
+    parsePacket v "act"    (\o -> ClientAct    <$> o .: "path")                 <|>
+    parsePacket v "edit"   (\o -> ClientEdit   <$> o .: "path" <*> o .: "text") <|>
+    parsePacket v "delete" (\o -> ClientDelete <$> o .: "path")                 <|>
+    parsePacket v "reply"  (\o -> ClientReply  <$> o .: "path" <*> o .: "text")
+
+{- Server -}
+
+data ServerPacket
+  = ServerHello [T.Text]
+  | ServerUpdate Path Node
+  deriving (Show)
+
+instance ToJSON ServerPacket where
+  toJSON (ServerHello extensions) =
+    object ["type" .= ("hello" :: T.Text), "extensions" .= extensions]
+  toJSON (ServerUpdate path node) =
+    object ["type" .= ("update" :: T.Text), "path" .= path, "node" .= node]
+
+  toEncoding (ServerHello extensions) =
+    pairs ("type" .= ("hello" :: T.Text) <> "extensions" .= extensions)
+  toEncoding (ServerUpdate path node) =
+    pairs ("type" .= ("update" :: T.Text) <> "path" .= path <> "node" .= node)
+
+instance FromJSON ServerPacket where
+  parseJSON v =
+    parsePacket v "hello" (\o -> ServerHello <$> o .: "extensions") <|>
+    parsePacket v "update" (\o -> ServerUpdate <$> o .: "path" <*> o .: "node")