Insect pests, diseases and other biotic agents have considerable impacts on forests. They can adversely affect tree growth, vigor and survival, the yield and quality of wood and non-wood products, wildlife habitat, recreation and the aesthetic appeal and cultural value of forests. They may also impede forest plantation programmes and make it necessary to abandon certain tree species or to clear cut large areas dominated by infested trees. This publication represents a rare effort to address forest pests and diseases comprehensively at the global level. Part I summarizes the results of a thematic study reviewing forest pests in 25 countries. Part II presents profiles of some globally important forest pest species, and III discusses select forest trees species and their associated pests. The information provided in this publication will assist forest health specialists, forest managers and policy-makers worldwide to make informed decisions.