Industrial Organization (IO) is the study of the structure of firms and their interactions in markets. A firm is an organization that buys resources to produce its products and then sells these products in a market in order to earn a profit. One of the important issues that IO addresses is how the structure of the market, such as the number of sellers, affects prices and profits of the firm(s) in the industry. This area is called market structure. Important models of market structure include markets with many firms (perfect competition), markets with a small number of firms (oligopoly), and markets with a single firm (monopoly).