In economics, specifically general equilibrium theory, a perfect market, also known as an atomistic market, is defined by several idealizing conditions, collectively called perfect competition, or atomistic competition. In theoretical models where conditions of perfect competition hold, it has been demonstrated that a market will reach an equilibrium in which the quantity supplied for every product or service, including labor, equals the quantity demanded at the current price. This equilibrium would be a Pareto optimum.

Perfect competition provides both allocative efficiency and productive efficiency:

• Such markets are allocatively efficient, as output will always occur where marginal cost is equal to average revenue i.e. price (MC = AR). In perfect competition, any profit-maximizing producer faces a market price equal to its marginal cost (P = MC). This implies that a factor's price equals the factor's marginal revenue product. It allows for derivation of the supply curve on which the neoclassical approach is based. This is also the reason why a monopoly does not have a supply curve. The abandonment of price taking creates considerable difficulties for the demonstration of a general equilibrium except under other, very specific conditions such as that of monopolistic competition.
• In the short-run, perfectly competitive markets are not necessarily productively efficient, as output will not always occur where marginal cost is equal to average cost (MC = AC). However, in the long-run, productive efficiency occurs as new firms enter the industry. Competition reduces price and cost to the minimum of the long run average costs. At this point, price equals both the marginal cost and the average total cost for each good (P = MC = AC).

The theory of perfect competition has its roots in late-19th century economic thought. Léon Walras gave the first rigorous definition of perfect competition and derived some of its main results. In the 1950s, the theory was further formalized by Kenneth Arrow and Gérard Debreu.

Imperfect competition was a theory created to explain the more realistic kind of market interaction that lies in between perfect competition and a monopoly. Edward Chamberlin wrote "Monopolistic Competition" in 1933 as "a challenge to the traditional viewpoint that competition and monopolies are alternatives and that individual prices are to be explained in either terms of one or the other" (Dewey,88.) In this book, and for much of his career, he "analyzed firms that do not produce identical goods, but goods that are close substitutes for one another" (Sandmo,300.)

Another key player in understanding imperfect competition is Joan Robinson, who published her book "The Economics of Perfect Competition" the same year Chamberlain published his. While Chamberlain focused much of his work on product development, Robinson focused heavily on price formation and discrimination (Sandmo,303.) The act of price discrimination under imperfect competition implies that the seller would sell their goods at different prices depending on the characteristic of the buyer to increase revenue (Robinson,204.) Joan Robinson and Edward Chamberlain came to many of the same conclusions regarding imperfect competition while still adding a bit of their twist to the theory. Despite their similarities or disagreements about who discovered the idea, both were extremely helpful in allowing firms to understand better how to center their goods around the wants of the consumer to achieve the highest amount of revenue possible.

Real markets are never perfect. Those economists who believe in perfect competition as a useful approximation to real markets may classify those as ranging from close-to-perfect to very imperfect. The real estate market is an example of a very imperfect market. In such markets, the theory of the second best proves that if one optimality condition in an economic model cannot be satisfied, it is possible that the next-best solution involves changing other variables away from the values that would otherwise be optimal.