Biological control is the suppression of pest populations by using predators, parasitoids, and pathogens. Historically, biological control has largely been used on a trial-and-error basis, and has failed more often than it has succeeded. However, by developing theories based on fundamental population principles and the biological characteristics of the pest and agent, we can gain a much better understanding of when and how to use biological control. This volume gathers recent theoretical developments and provides a balanced guide to the important issues that need to be considered when applying ecological theory to biological control.