Economic profit does not occur in perfect competition in long run equilibrium; if it did, there would be an incentive for new firms to enter the industry, aided by a lack ofbarriers to entry until there was no longer any profit.^[2] As new firms enter the industry, they increase the supply of the product available in the market, and these new firms are forced to charge a lower price to entice consumers to buy the additional supply these new firms are supplying (they compete for customers).^[4][5][6][7] Incumbent firms within the industry face losing their existing customers to the new firms entering the industry, and are therefore forced to lower their prices to match the lower prices set by the new firms. New firms will continue to enter the industry until the price of the product is lowered to the point that it is the same as the average cost of producing the product, and all of the economic profit disappears.^[4][5] When this happens, economic agents outside of the industry find no advantage to entering the industry, supply of the product stops increasing, and the price charged for the product stabilizes.^[4][5][6]

The same is likewise true of the long run equilibria of monopolistically competitive industries and, more generally, any market which is held to be contestable. Normally, a firm that introduces a differentiated product can initially secure market power for a short while. At this stage, the initial price the consumer must pay for the product is high, and the demand for, as well as the available of the product in the market, will be limited. In the long run, however, when the profitability of the product is well established, and because there are few barriers to entry.^[4][5][6] The number of firms that produce this product will increase until the available supply of the product eventually becomes relatively large, the price of the product shrinks down to the level of the average cost of producing the product. When this finally occurs, all monopoly associated with producing and selling the product disappears, and the initial monopoly turns into a competitive industry.^[4][5][6] In the case of contestable markets, the cycle is often ended with the departure of the former "hit and run" entrants to the market, returning the industry to its previous state, just with a lower price and no economic profit for the incumbent firms.

Profit can, however, occur in competitive and contestable markets in the short run, as firms jostle for market position. Once risk is accounted for, long-lasting economic profit in a competitive market is thus viewed as the result of constant cost-cutting and performance improvement ahead of industry competitors, allowing costs to be below the market-set price.