The document discusses oligopolies, which are market structures with only a few firms. It notes that oligopolies exist between perfect competition and pure monopoly. Oligopolies are characterized by interdependent firms that would benefit from cooperation to behave like a monopolist, but finding and maintaining cooperation can be difficult due to tensions between self-interest and joint interests. The document uses examples like duopolies, cartels, and game theory models to illustrate challenges around cooperation in oligopolistic markets.