Caligula (; 31 August 12 – 24 January 41), formally known as Gaius (Gaius Caesar Augustus Germanicus), was the third Roman emperor, ruling from AD 37 until his assassination in AD 41. He was the son of the Roman general Germanicus and Agrippina the Elder, Augustus' granddaughter. Caligula was born into the first ruling family of the Roman Empire, conventionally known as the Julio-Claudian dynasty.

Although Gaius was named after Gaius Julius Caesar, he acquired the nickname "Caligula" ("little caliga", a type of military boot) from his father's soldiers during their campaign in Germania. When Germanicus died at Antioch in 19, Agrippina returned with her six children to Rome, where she became entangled in a bitter feud with Tiberius. The conflict eventually led to the destruction of her family, with Caligula as the sole male survivor. In 26, Tiberius withdrew from public life to the island of Capri, and in 31, Caligula joined him there. Following the former's death in 37, Caligula succeeded him as emperor. There are few surviving sources about the reign of Caligula, though he is described as a noble and moderate emperor during the first six months of his rule. After this, the sources focus upon his cruelty, sadism, extravagance, and sexual perversion, presenting him as an insane tyrant.

While the reliability of these sources is questionable, it is known that during his brief reign, Caligula worked to increase the unconstrained personal power of the emperor, as opposed to countervailing powers within the principate. He directed much of his attention to ambitious construction projects and luxurious dwellings for himself, and he initiated the construction of two aqueducts in Rome: the Aqua Claudia and the Anio Novus. During his reign, the empire annexed the client kingdom of Mauretania as a province. In early 41, Caligula was assassinated as a result of a conspiracy by officers of the Praetorian Guard, senators, and courtiers. However, the conspirators' attempt to use the opportunity to restore the Roman Republic was thwarted. On the day of the assassination of Caligula, the Praetorians declared Caligula's uncle, Claudius, as the next emperor. Caligula's death marked the official end of the Julii Caesares in the male line, though the Julio-Claudian dynasty continued to rule until the demise of his nephew, Nero.