The Animagus Registry is a public record maintained by the Ministry of Magic that lists all witches and wizards who have legally registered their Animagus abilities. The primary purpose of the registry is to ensure that these powerful individuals do not abuse their abilities for illicit or criminal purposes, as the skill can be used for spying and other forms of deception. For each registered Animagus, the registry contains specific and crucial details, including:
A canonical example is the entry for Minerva McGonagall, which notes that her tabby cat form has distinctive square spectacle markings around its eyes. This level of detail allows the Ministry of Magic to identify a registered Animagus even when they are transformed.
According to Wizarding law, it is mandatory for any witch or wizard who acquires the ability to become an Animagus to register themselves with the Ministry of Magic. The process of becoming an Animagus is exceptionally difficult and requires great skill in Transfiguration. Failure to register is considered a serious crime, and those who are discovered to be unregistered Animagi face severe penalties, including imprisonment in Azkaban. According to research conducted by Hermione Granger in the 1993-1994 Hogwarts school year, there were only seven legally registered Animagi in the entire twentieth century.
The Animagus Registry plays a subtle but critical role in the series, primarily as a tool for investigation and leverage. In Harry Potter and the Prisoner of Azkaban, Hermione Granger consults the registry while investigating how Sirius Black could have possibly entered Hogwarts undetected. She discovered that there were only seven registered Animagi that century and that Sirius Black was not on the list. While this led her to a dead end at the time, it established the existence and rules of the registry, which later proved vital in understanding that Black, along with James Potter and Peter Pettigrew, became Animagi illegally to support their friend Remus Lupin. In Harry Potter and the Goblet of Fire, Hermione's knowledge of the registry becomes a powerful weapon. After deducing that the intrusive journalist Rita Skeeter is an unregistered beetle Animagus, Hermione captures her in a jar. She then uses the threat of exposing Skeeter to the Ministry of Magic—and the subsequent punishment of imprisonment in Azkaban—to force her to cease writing slanderous articles for the Daily Prophet.