Harmodius and Aristogeiton were two lovers in Classical Athens who became known as the Tyrannicides for their assassination of Hipparchus, the brother of the tyrant Hippias, for which they were executed. A few years later, in 510 BC, the Spartan king Cleomenes I forced Hippias to go into exile, thereby opening the way to the subsequent democratic reforms of Cleisthenes. The Athenian democrats later celebrated Harmodius and Aristogeiton as national heroes, partially to conceal the role played by Sparta in the removal of the Athenian tyranny. Cleisthenes notably commissioned the famous statues of the Tyrannicides.

Tyrannicide or tyrannomachia is the killing or assassination of a tyrant or unjust ruler, purportedly for the common good, and usually by one of the tyrant's subjects. Tyrannicide was legally permitted and encouraged in Classical Greece. Often, the term "tyrant" was a justification for political murders by rivals, but in some exceptional cases students of Platonic philosophy risked their lives against tyrants. The killing of Clearchus of Heraclea in 353 BC by a cohort led by his own court philosopher is considered a sincere tyrannicide. A person who carries out a tyrannicide is also called a "tyrannicide".