Oligopoly is the least understood market structure; consequently, it has no single, unified theory. Nevertheless, there is some agreement as to what constitutes an oligopolistic market. Three conditions for oligopoly have been identified. First, an oligopolistic market has only a few large firms. This condition distinguishes oligopoly from monopoly, in which there is just one firm. Second, an oligopolistic market has high barriers to entry. This condition distinguishes oligopoly from perfect competition and monopolistic competition in which there are no barriers to entry. Third, oligopolistic firms may produce either differentiated or homogeneous products. Examples of oligopolistic firms include automobile manufacturers, oil producers, steel manufacturers, and passenger airlines.