Imperfect competition or imperfectly competitive markets is one in which some of the rules of *perfect competition* are not followed. Virtually, all real world markets follow this model, as in practice, all markets have some form of *imperfection*. When dealing with imperfect competition the *equilibrium* price can be influenced by the actions of agents. In imperfect competition the price of goods can increase above their *marginal cost* and thus have customers decrease their level of purchase, and so reach inefficient levels of *production*. *Governments* try to avoid these situations and take measures to stop imperfect competition.

The most common forms of imperfect competition include: *monopolies*, *oligopolies*, *duopolies*, *monopolistic competition* and *monopsony*.

Roy Harrod was the first economist to develop the theory of imperfect competition and, other authors, such as *Edward Chamberlin* and *Joan Robinson* renewed its interest and made major contributions. Nevertheless, it is important to point out that *Cournot*, in his “Researches into the Mathematical Principles of the Theory of Wealth”, 1838, was the first to model this kind of markets.