Firms that operate in non-perfectly-competitive markets (monopoly, oligopoly, and monopolistic competition) have market power and, thus, the ability to set prices. In these markets, the firms will set prices at profit-maximizing levels by first finding their profit-maximizing output level and then setting a price such that the number of units that will be demanded will equal the profit-maximizing output level.

In perfect competition, on the other hand, competition is strong and each firm sells an identical product. Because of this, no individual firm has any influence in the market. This leads to all firms being price takers. In this circumstance, the price is set by the interaction of supply and demand in the market and is equal to the equilibrium price. Each firm then sells all of their output at the market-determined price.