A counterculture is a culture whose values and norms of behavior are opposed to those of the current mainstream society, and sometimes diametrically opposed to mainstream cultural mores. A countercultural movement expresses the ethos and aspirations of a specific population during a well-defined era. When oppositional forces reach critical mass, countercultures can trigger dramatic cultural changes.

Prominent examples of countercultures in the Western world include the Levellers (1645–1650), Bohemianism (1850–1910), the more fragmentary counterculture of the Beat Generation (1944–1964), and the globalized counterculture of the 1960s which in the United States consisted primarily of Hippies and Flower Children (c. 1965–1973, peaking in 1967–1970). Regarding this last group, when referring to themselves, counterculture will usually be capitalized and is often hyphenated as: Counter-Culture or Counter-culture.

John Milton Yinger originated the term "contraculture" in his 1960 article in American Sociological Review. Yinger suggested the use of the term contraculture "wherever the normative system of a group contains, as a primary element, a theme of conflict with the values of the total society, where personality variables are directly involved in the development and maintenance of the group's values, and wherever its norms can be understood only by reference to the relationships of the group to a surrounding dominant culture."

Some scholars have attributed the counterculture to Theodore Roszak, author of The Making of a Counter Culture. It became prominent in the news media amid the social revolution that swept the Americas, Western Europe, Japan, Australia, and New Zealand during the 1960s.

Scholars differ in the characteristics and specificity they attribute to "counterculture". "Mainstream" culture is also difficult to define, and in some ways becomes identified and understood through contrast with counterculture. Counterculture might oppose mass culture (or "media culture"), or middle-class culture and values. Counterculture is sometimes conceptualized in terms of generational conflict and rejection of older or adult values.

Counterculture may or may not be explicitly political. It typically involves criticism or rejection of currently powerful institutions, with accompanying hope for a better life or a new society. It does not look favorably on party politics or authoritarianism.

Cultural development can also be affected by way of counterculture. Scholars such as Joanne Martin and Caren Siehl, deem counterculture and cultural development as "a balancing act, [that] some core values of a counterculture should present a direct challenge to the core values of a dominant culture". Therefore, a prevalent culture and a counterculture should coexist in an uneasy symbiosis, holding opposite positions on valuable issues that are essentially important to each of them. According to this theory, a counterculture can contribute a plethora of useful functions for the prevalent culture, such as "articulating the foundations between appropriate and inappropriate behavior and providing a safe haven for the development of innovative ideas".

During the late 1960s, hippies became the largest and most visible countercultural group in the United States.