The Marauder's Map is a magical item in the Harry Potter novels, taking the form of a map of Hogwarts which reveals the location of everyone inside the castle, along with secret passageways. Given the large scale of Hogwarts, the map represents quite a feat, especially as certain parts of the castle, such as the moving stairways, seem to change at whim. The map originally appears in Harry Potter and the Prisoner of Azkaban and plays an important role in that and every subsequent book.
Harry's father, James Potter, was one of the makers of the Marauder's Map. The Marauder's Map appears to have been inspired by the nighttime wanderings of the friends James Potter, Sirius Black, Peter Pettigrew, and Remus Lupin, a werewolf, and represents mastery of highly advanced magic. Clearly Potter, Black, and Pettigrew are capable of skilled magic, as the three became Animagi so that they could accompany Lupin when he transformed into a werewolf. As Animagi, the three boys were capable of changing into various animals, so that Lupin in his wolf guise would not attack them.
The highly illegal and dangerous act of becoming Animagi appears to have served as a bond for the boys while at school, and the group of four called themselves the Marauders, making large amounts of mischief at Hogwarts while exploring the school and grounds. They also gave themselves nicknames: Lupin was called Moony, because his werewolf identity, and the others were named after the animals they could transform into. Potter was known as Prongs, Pettigrew as Wormtail, and Black as Padfoot.
The Marauder's Map, like many magical items, retains some of the character of its creators. When a witch or wizard taps the map with a wand and says “I solemnly swear that I am up to no good,” the map activates, drawing a detailed map of Hogwarts and pinpointing the locations of various individuals in the castle. This property of the Marauder's Map helps Harry Potter evade teachers who might punish him for being in the corridors after curfew and it also reveals anomalies, such as Peter Pettigrew, who is supposed to be dead. When the Marauder's Map has served its purpose, another wand tap, accompanied by “mischief managed,” causes it to return to its latent state, making it appear to be a scrap piece of parchment. If someone, such as a teacher, does not know how to work the map, it responds in the way it deems most appropriate, insulting Severus Snape in one of the books, and possibly telling Fred and George how to activate it when they first discovered it in the office of Argus Filch.
In addition to showing secret passages, the Marauder's Map will also show the user how to access them. A small animation showing which incantation is needed to open a door will appear if the user is in need. The Marauder's Map does appear to have some flaws. It does not, for example, show the Room of Requirement, probably because the room is Unplottable, meaning that it cannot appear on any map. Hogwarts, however, is also Unplottable, so it is unclear how the Marauder's found a workaround for this magical protection. In addition, the Marauder's Map cannot distinguish between two people of the same name, which turns out to be a problem in Harry Potter and the Goblet of Fire, when Bartimeus Crouch, Jr. is wandering the castle, but is identified simply as Bartimeus Crouch, leading to some confusion.