A situation of oligopoly meets when on a Marché there are a very low number of offerers (salesmen) and a big number of applicants (customers). One also speaks about oligopolistic situation of walked .

It is about a situation of imperfect market: within the framework of the Pure competition and perfect, the offerers are independent, whereas in the case of an oligopoly the profit of each producer depends on the attitude of the other offerers.

When the outputs are constant or increasing, the producers are naturally incited to grow bigger in order to carry out economies of scale, which tends to the concentration, and balances it of such a system is a situation of Monopole where there remains nothing any more but one producer. Nevertheless, in the aim of protection the consumer of the abuses, the political institutions are opposed to the constitution monopolies. These markets tend then from now on naturally towards an oligopolistic balance.

Once this balance reached, the producers can deliver a fierce competition (case of Intel and AMD on the market of the Microprocesseur S), but can also get along in a more or less formal way and constitute a Cartel. Just as they prohibit the dominant position abuse, the political institutions are opposed to the abusive agreements. For example in France, there exist three companies (offerers) which propose telephone services mobile to million applicants (customers). Competition becoming quasi-non-existent between the operators, the French State intervened in order to oblige the offerers to revise the cost of the SMS (Short Message Service) which were invoicees six to eight times their cost price.

The simplest case of the oligopoly is a Duopole, where there are two producers.

Some Branches of industry are “naturally oligopolistic” sectors: the outputs of scale are so large that it is more profitable for the economy than the number of actors is limited (see also natural Monopole).