Patronage (clientela) was the distinctive relationship in ancient Roman society between the patronus ('patron') and their cliens ('client'). Apart from the patron-client relationship between individuals, there were also client kingdoms and tribes, whose rulers were in a subordinate relationship to the Roman state.

The relationship was hierarchical, but obligations were mutual. The patron was the protector, sponsor, and benefactor of the client; the technical term for this protection was patrocinium. Although typically the client was of inferior social class, a patron and client might even hold the same social rank, but the former would possess greater wealth, power, or prestige that enabled him to help or do favors for the client.

From the emperor at the top to the commoner at the bottom, the bonds between these groups found formal expression in the legal definition of patrons' responsibilities to clients. Patronage relationships were not exclusively between two people and also existed between a general and his soldiers, a founder and colonists, and a conqueror and a dependent foreign community.

Benefits a client may be granted include legal representation in court, loans of money, influencing business deals or marriages, and supporting a client's candidacy for political office or a priesthood. Arranging marriages for their daughters, clients were often able to secure new patrons and extend their influence in the political arena. In return for these services, the clients were expected to offer their services to their patron as needed. A client's service to the patron included accompanying the patron in Rome or when he went to war, ransoming him if he was captured, and supporting him during political campaigns.

Requests were usually made by clientela at a daily morning reception at the patron's home, known as the salutatio. The patron would receive his clients at dawn in the atrium and tablinum, after which the clients would escort the patron to the forum. The number of clients who accompanied their patron was seen as a symbol of the patron's prestige. The client was regarded as a minor member of their patron's gens, entitled to assist in its sacra gentilicia, and bound to contribute to the cost of them. The client was subject to the jurisdiction and discipline of the gens, and was entitled to burial in its common sepulchre.

One of the major spheres of activity within patron–client relations was the law courts, but clientela was not itself a legal contract, although it was supported by law from earliest Roman times. The pressures to uphold one's obligations were primarily moral, founded on ancestral custom, and on qualities of good faith on the part of the patron and loyalty on the part of the client. The patronage relationship was not a discrete one, but a network, since a patronus might himself be obligated to someone of higher status or greater power. A client might have more than one patron, whose interests could come into conflict. While the Roman familia ('family', but more broadly the "household") was the building block of society, interlocking networks of patronage created highly complex social bonds.

Reciprocity ethics played a major role in the patron-client system. Favors given from patron to client and client to patron do not cancel each other; instead, the giving of favors and counter favors was symbolic of the personal relationship between patron and client. As a consequence, the act of returning a favor was done more out of a sense of gratuity and less so because a favor needed to be returned.

The regulation of the patronage relationship was believed by the Greek historians Dionysius and Plutarch to be one of the early concerns of Romulus. Hence, it was dated to the very founding of Rome. In the earliest periods, patricians would have served as patrons. Both patricius, 'patrician', and patronus are related to the Latin word pater, 'father', in this sense symbolically, indicating the patriarchal nature of Roman society. Although other societies have similar systems, the patronus cliens relationship was "peculiarly congenial" to Roman politics and the sense of familia in the Roman Republic. An important person demonstrated their prestige or dignitas by the number of clients they had.