Manumission, or enfranchisement, is the act of freeing slaves by their owners. Different approaches to manumission were developed, each specific to the time and place of a particular society. Historian Verene Shepherd states that the most widely used term is gratuitous manumission, "the conferment of freedom on the enslaved by enslavers before the end of the slave system".

The motivations for manumission were complex and varied. Firstly, it may present itself as a sentimental and benevolent gesture. One typical scenario was the freeing in the master's will of a devoted servant after long years of service. A trusted bailiff might be manumitted as a gesture of gratitude. For those working as agricultural labourers or in workshops, there was little likelihood of being so noticed. In general, it was more common for older slaves to be given freedom.

Legislation under the early Roman Empire put limits on the number of slaves that could be freed in wills (lex Fufia Caninia, 2 BC), which suggests that it had been widely used. Freeing slaves could serve the pragmatic interests of the owner. The prospect of manumission worked as an incentive for slaves to be industrious and compliant. Manumission contracts, found in some abundance at Delphi (Greece), specify in detail the prerequisites for liberation.

A History of Ancient Greece explains that in the context of Ancient Greece, affranchisement came in many forms. A master choosing to free his slave would most likely do so only "at his death, specifying his desire in his will". In rare cases, slaves who were able to earn enough money in their labour were able to buy their own freedom and were known as choris oikointes. Two 4th-century bankers, Pasion and Phormion, had been slaves before they bought their freedom. A slave could also be sold fictitiously to a sanctuary from where a god could enfranchise him. In very rare circumstances, the city could affranchise a slave. A notable example is that Athens liberated everyone who was present at the Battle of Arginusae (406 BC).

Even once a slave was freed, he was not generally permitted to become a citizen, but would become a metic. The master then became the metic's prostatès (guarantor or guardian). The former slave could be bound to some continuing duty to the master and was commonly required to live near the former master (paramone). Ex-slaves were able to own property outright, and their children were free of all constraint.

Under Roman law, a slave had no personhood and was protected under law mainly as his or her master's property. A slave who had been manumitted was a libertus (feminine liberta) and a citizen. Manumissions were subject to a state tax.

The soft felt pileus hat was a symbol of the freed slave and manumission; slaves were not allowed to wear them:

Among the Romans the cap of felt was the emblem of liberty. When a slave obtained his freedom he had his head shaved, and wore instead of his hair an undyed pileus (πίλεον λευκόν, Diodorus Siculus Exc. Leg. 22 p625, ed. Wess.; Plaut. Amphit. I.1.306; Persius, V.82). Hence the phrase servos ad pileum vocare is a summons to liberty, by which slaves were frequently called upon to take up arms with a promise of liberty (Liv. XXIV.32). "The figure of Liberty on some of the coins of Antoninus Pius, struck A.D. 145, holds this cap in the right hand".