Insect pests damage millions of hectares of forest worldwide each year. Moreover, the extent of such damage is increasing as international trade grows, facilitating the spread of insect pests, and as the impacts of climate change become more evident. Classical biological control is a well-tried, cost-effective approach to the management of invasive forest pests. It involves the importing of natural enemies of non-native pests from their countries of origin with the aim of establishing permanent, self-sustaining populations capable of sustainably reducing pest populations below damaging levels. A great deal of knowledge on classical biological control has been accumulated worldwide in the last few decades. This publication, which was written by a team of experts, distils that information in a clear, concise guide aimed at helping forest-health practitioners and forest managers - especially in developing countries - to implement successful classical biological control programmes. It provides general theory and practical guidelines, explains the why and how of classical biological control in forestry, and addresses the potential risks associated with such programmes. It features 11 case studies of successful efforts worldwide to implement classical biological control.