Oligopoly is a market structure with a small number of firms producing either homogeneous or differentiated products. Firms in an oligopoly are interdependent, so the actions of one firm affect others. Firms may collude to act like a monopoly and earn profits, or they may compete and drive prices down to zero profits. Collusion is more difficult to sustain when price cuts are hard to detect, market conditions are unstable, and antitrust enforcement is vigorous. Some oligopolies involve price leadership, where one firm sets the industry price that others follow.