Many species of natural enemies are commercially mass-reared and released to control arthropod pests in systems ranging from home gardens to large-scale agricultural operations. However, limited efforts have been made in North America to assess the rationale and conditions for their use systematically, as well as the extent of their use. This chapter reviews 4 cases in which the practical use of mass-reared natural enemies, including parasitoids, predators, and/or entomopathogenic nematodes, can be documented. In British Columbia, ≥1 species of natural enemy are used in almost all greenhouse vegetable production; in California, a mass-reared parasitoid for control of scale is released on a major portion of citrus each year; also in California, 70–85% of the strawberry growers use predaceous mites; and a considerable proportion of U.S. cranberry growers use entomopathogenic nematodes. In each of these cases, high-value crops are involved and the pesticide control options are very limited. Also, mass-reared natural enemies have been used in each case within the context of holistic integrated pest management (IPM) programs. With some exceptions, these IPM programs include timely pest and natural enemy monitoring, action thresholds, the use of several nonchemical pest management tactics, and the selective use of pesticides. An additional key component to the successful use of mass-reared natural enemies has been a close working relationship among end-users, consultants, grower associations, producers of natural enemies, researchers, and extension personnel. Day-to-day guidance, accurate practical information, and maintenance of good relations between natural enemy suppliers and users are especially important. Although these case studies provide considerable insight into the successful use of mass-reared natural enemies, additional research and development efforts, particularly to reduce costs of production and application, are needed. Likewise, continued close cooperation among a variety of public and private institutions will be essential to ensure the continued and expanded use of mass-reared natural enemies as a viable pest management tool.