Oligarchy is a form of government where most political power effectively rests with a small segment of society (typically the most powerful, whether by wealth, military strength, ruthlessness, or political influence). The word oligarchy is from the Greek for "few" and "rule". Some political theorists have argued that all societies are inevitably oligarchies no matter the supposed political system.

Oligarchies are often controlled by a few powerful families whose children are raised and mentored to become inheritors of the power of the oligarchy, often at some sort of expense to those governed. In contrast to aristocracy ("government by the 'best'"), this power may not always be exercised openly, the oligarchs preferring to remain "the power behind the throne", exerting control through economic means. Unlike plutocracy, oligarchy is not always a rule by wealth, as oligarchs can simply be a privileged cadre. It has also been suggested that most communist states fit the definition of oligarchy.

A society may become an oligarchy by default as an outgrowth of the shifting alliances of warring tribal chieftans, although any form of government may transform into an oligarchy at some point in its evolution. The most likely mechanism for this transformation is a gradual accumulation of otherwise unchecked economic power. Oligarchies may also evolve into more classically authoritarian forms of government, sometimes as the result of one family gaining ascendancy over the others. Many of the European monarchies established during the late Middle Ages began in this way.

Oligarchies may also become instruments of transformation, insisting that monarchs or dictators share power, thereby opening the door to power-sharing by other elements of society. One example of this process occurred when English nobles banded together in 1215 to force a reluctant King John of England to sign the Magna Carta, a tacit recognition both of King John's waning political power and of the existence of an incipient oligarchy. As English society continued to grow and develop, the Magna Carta was repeatedly revised (1216,1217, and 1225), guaranteeing greater rights to greater numbers of people, thus setting the stage for British constitutional monarchy.

A modern example of oligarchy could be seen in South Africa during the 20th century. Here, the basic characteristics of oligarchy are particularly easy to observe, since the South African form of oligarchy was based on racism. After the Boer War, a tacit agreement was reached between English- and Afrikaans-speaking whites. Together, they made up about twenty percent of the population, but this small percentage had access to virtually all the educational and trade opportunities, and they proceeded to deny this to the black majority even further than before. Although this process had been going on since the mid-18th century, after 1948 it became official government policy and became known worldwide as apartheid. This lasted until the arrival of democracy in South Africa in 1994, punctuated by the transition to a democratically-elected government dominated by the black majority.