This chapter describes how prescription drug markets deviate from “normal” competitive markets. On the demand side, there is considerable uncertainty in disease prognosis and treatment effects; buyers have much less information than sellers; consumers usually do not pay directly for their prescriptions but rather have health insurance, which protects them from a drug’s full price; and physicians and insurers play a major role in consumer choices. On the supply side, firms are heavily regulated, subject to laws requiring that drugs undergo extensive testing before entering the marketplace. Monopolies are a system feature, designed to incentivize companies to invest in expensive and risky drug discovery by providing the prospect of a big payoff if the investment succeeds. Patents and market exclusivity restrict competition and provide drug developers considerable pricing power. The challenge for policymakers is how to achieve reasonable or fair drug prices in light of these market distortions.