An oligopoly is a market structure with a small number of producers that have strategic interdependence. Each firm must consider how its rivals will respond to its pricing and output decisions. While firms have an incentive to collude to raise prices above competitive levels, maintaining collusion can be difficult without formal agreements. As more firms enter the market, it becomes harder to collude and prices tend to approach competitive levels. Firms in an oligopoly often engage in tacit collusion through mechanisms like price leadership.