The Julio-Claudian dynasty comprised the first five Roman emperors: Augustus, Tiberius, Caligula, Claudius, and Nero.

This line of emperors ruled the Roman Empire, from its formation (under Augustus, in 27 BC) until the last of the line, Emperor Nero, committed suicide (in AD 68).

The name Julio-Claudian is a historiographical term, deriving from the two families composing the imperial dynasty: the Julii Caesares and Claudii Nerones.

Julius and Claudius were two Roman family names; in classical Latin, they came second. Roman family names were inherited from father to son, but a Roman aristocrat could—either during his life or in his will—adopt an heir if he lacked a natural son. In accordance with Roman naming conventions, the adopted son would replace his original family name with the name of his adopted family. A famous example of this custom is Julius Caesar's adoption of his great-nephew, Gaius Octavius.

Primogeniture is notably absent in the history of the Julio-Claudian dynasty. Augustus, Caligula and Nero failed to father biological and legitimate sons. Tiberius' own son, Drusus predeceased him. Only Claudius was outlived by his son, Britannicus, although he opted to promote his adopted son Nero as his successor to the throne. Adoption ultimately became a tool that most Julio-Claudian emperors utilized in order to promote their chosen heir to the front of the succession. Augustus—himself an adopted son of his great-uncle, the Roman dictator Julius Caesar—adopted his stepson Tiberius as his son and heir. Tiberius was, in turn, required to adopt his nephew Germanicus, the father of Caligula and brother of Claudius. Caligula adopted his cousin Tiberius Gemellus (grandson of the emperor Tiberius) shortly before executing him. Claudius adopted his great-nephew and stepson Nero, who, lacking a natural or adopted son of his own, ended the reign of the Julio-Claudian dynasty with his fall from power and subsequent suicide.

Augustus (Imperator Caesar Divi Filius Augustus), as Caesar's adopted son and heir, discarded the family name of his natural father and initially renamed himself "Gaius Julius Caesar" after his adoptive father. It was also customary for the adopted son to acknowledge his original family by adding an extra name at the end of his new name. As such, Augustus' adopted name would have been "Gaius Julius Caesar Octavianus". However, there is no evidence that he ever used the name Octavianus.

Following Augustus' ascension as the first emperor of the Roman Empire in 27 BC, his family became a de facto royal house, known in historiography as the "Julio-Claudian dynasty". For various reasons, the Julio-Claudians followed in the example of Julius Caesar and Augustus by utilizing adoption as a tool for dynastic succession. The next four emperors were closely related through a combination of blood relation, marriage and adoption.^{[citation needed]}

Tiberius (Tiberius Caesar Divi Augusti Filius Augustus), a Claudian by birth, became Augustus' stepson after the latter's marriage to Livia, who divorced Tiberius' natural father in the process. Tiberius' connection to the Julian side of the Imperial family grew closer when he married Augustus' only daughter, Julia the Elder. He ultimately succeeded Augustus as emperor in AD 14 after becoming his stepfather's adopted son and heir.