For some U.S. cities, a good deal is known about who retails cocaine and heroin, the size and stability of organizations in which retailers work, and drug prices and incomes. A much smaller body of research, however, has examined various aspects of high-level drug trafficking. Some of the principal findings of research on drug markets are reviewed that indicate monopoly control is rare, retail employees have very low earnings, the structure and conduct of drug markets varies across space and time, immigrants play an important role, and retailing is characterized by low trust. The author identifies three motives for research on drug markets--curiosity, scientific interest, and the need to raise the effectiveness of drug enforcement. An agenda for research on drug markets and policy formulation is briefly described that focuses on reducing drug use, drug selling, and associated harms. 23 references and 6 notes