Moon Dungeons are final dungeons from The Legend of Zelda: Majora's Mask. Located at the Moon, there are four Moon Dungeons in all. The puzzles in each of the four dungeons are designed for each of the four forms Link can assume in the game: Deku, Goron, Zora and Hylian. Completion of the Moon Dungeons is completely optional, but if Link hopes to obtain the Fierce Deity's Mask before going into battle with Majora, he must complete all four.
When Link manages to break Majora's hold on the Skull Kid, he chases after it to the Moon. Here, Link finds a grassy plain with a tree in the center. Running around the tree are four Lunar Children wearing the boss masks, while a child wearing Majora's Mask sits under the tree alone.
If Link speaks with one of the Lunar Children running about, the child will ask him to participate in a game of Hide-and-Seek. If Link accepts he will have to give up a certain number of masks, and he will be transported to a mysterious dungeon themed around one of his transformations; the Child wearing Odolwa's mask warps Link to the Deku dungeon, the Goht child takes Link to the Goron dungeon, the child wearing Gyorg's mask results in the Zora dungeon, and the child donning Twinmold's mask will bring Link to the Hylian dungeon. In each of these mini-dungeons, Link must overcome a series of obstacles dealing directly with the designated transformation — for example, the Goron Moon Dungeon is heavily based around the concept of Goron Link's rolling abilities. Each dungeon contains a Piece of Heart and several Gossip Stones that subtly reveal the locations of all of the game's masks.
When Link completes a mini-dungeon, he will reach the mysterious playroom of the respective Lunar Child. In the room, the walls are covered with colorful wallpaper and toy blocks are found scattered about. Here, Link will have to give up more masks, and the Lunar Child will ask Link a mysterious question. Before the hero can respond, however, he is transported back to the grassy field. Once Link successfully completes all of the Moon Dungeons, he can speak with the youth wearing Majora's Mask to obtain the Fierce Deity's Mask and go into the final battle with Majora.