Oligarchy (from Greek ὀλιγαρχία (oligarkhía) 'rule by few'; from ὀλίγος (olígos) 'few', and ἄρχω (arkho) 'to rule or to command') is a conceptual form of power structure in which power rests with a small number of people. These people may or may not be distinguished by one or several characteristics, such as nobility, fame, wealth, education, or corporate, religious, political, or military control.

Throughout history, power structures considered to be oligarchies have often been viewed as tyrannical, relying on public obedience or oppression to exist. Aristotle pioneered the use of the term as meaning rule by the rich, for which another term commonly used today is plutocracy. One of the first oligarchies in history is that of Sparta, which developed the concept alongside its rival Athens, and essentially provided a counterpoint to Athenian democracy. In the early 20th century Robert Michels developed the theory that democracies, like all large organizations, tend to turn into oligarchies. In his "Iron law of oligarchy" he suggests that the necessary division of labor in large organizations leads to the establishment of a ruling class mostly concerned with protecting their own power.