A major concern for biological control has always been the risk of indirect unwanted effects on the ecology of other organisms. Our understanding of the ecological and evolutionary processes underlying these effects has until now been limited and experimental methods sometimes lacking. This book presents the key papers from of the first International Organisation for Biological Control global symposium, held in Montpellier, France, in October 1999. It addresses the issues and concerns involved in biological control, and assesses the current status of evaluation of the ecological effects.