Real estate appraisers represent a unique and very interesting form of business organization characterized by both cooperation and competition that is quite unlike any of the normal models of competition (such as perfect competition, oligopoly, or monopoly) that were studied in introductory economic theory courses. The interesting part for an economist is that appraisers are competitors and yet seem to depend on each other. Appraisers are clearly competitors; that is, they all compete for the same set of business opportunities available in the marketplace, just like farmers or barbers or dentists compete for a share of their perspective markets. At the same time, appraisers cooperate through the sharing of information about comparable sales, both through informal networks as well as formally through a sales data bank.