Insect pests and diseases routinely affect the health of trees, and major outbreaks can have catastrophic environmental and economic impacts. The effective management of all tree health problems depends on their early detection. This guide will help readers recognize symptoms of ill health in trees and understand their general significance. The guide provides 140 photographs of symptoms from more than 50 tree species to be used as a basis for demonstrating the effects of pest (biotic) and non-living (abiotic) influences on trees. It is not and identification guide to insects pests and diseases of trees, but aims to help people make visual assessments of tree health problems and to provide a preliminary diagnosis which will in turn assist in better planning and more effective management of forests and trees. The guide will be useful to anyone interested in tree health or responsible for managing trees.