The introduction of foreign parasites and predators has resulted in a significant degree of sustained, self-perpetuating control of more than 100 insect pests over the world. In at least half of these cases, the degree of control has been great enough to render all other control practices unnecessary. Successful biological control has been achieved in many countries and over a broad range of geographic and climatic conditions. Those regions in which the greatest numbers of successes have been achieved are where a sustained effort has been made. Although the successes in the introduction of exotic agents for permanent biological control have been many, the failures have been more. Most failures probably resulted because ecological information was incomplete or ignored. The current trend is towards a more careful consideration of these factors before attempting introductions.