Parasitoids are insects that parasitize and eventually kill other insects. Between one and two million species of parasitoid insect exist on the earth today. This book explores how this staggering diversity is maintained and documents patterns in host-parasitoid interactions, including parasitoid community richness, the importance of parasitoids as mortality factors, and their impact on host densities as determined by the outcomes of parasitoid introductions for biological control. It documents general patterns using data sets generated from the global literature and evaluates potential underlying biological, ecological, and evolutionary mechanisms. A theme running throughout the book is the importance of host refuges as a major constraint on host-parasitoid interactions.