Oligopoly market structure takes a number of different forms. There are collusive models, where participants actively agree to some form of price or quantity fixing (OPEC – quotas for oil; non-collusive); price reaction theories, where participants don’t actively conspire to fix anything, but anticipate reactions from others – such as the theory of price stability called the Kinked Demand Curve Theory, or the game theories based on the prisoner’s dilemma (theories of probable outcomes). Here isa  diagram summarising the main branches of oligopoly theory.