The technological, institutional, and human factors known to affect the organization of legal markets are also relevant to the understanding of how the markets for illegal products are organized. The illegality of a product results in the threat of police intervention and in the lack of contracts that are enforceable in court. These factors lead to the formation of small and relatively transitory enterprises. The possibility that participants will provide information about others to the police so that their penalties will be reduced is the critical mediating factor in these markets. Corrupt police do not change this result unless there is a single police organization with sole jurisdiction. Little collusion exists in these markets even though no antitrust authorities exist. In addition, several factors limit the role of violence in organizing illegal markets. These and other characteristics of illegal markets suggest that enforcement campaigns aimed at illegal markets are unlikely to be successful except when they attack organized crime in areas where its power is already strong or its involvement is growing. Footnotes, 38 references.