Since the beginning of the 80s, there has been increasing awareness of the threat to our forests which makes forest damage and in particular its visible symptoms an important topic. This book summarizes how trees react to changes in the environment and to stress. Illustrated examples of the great variety in appearance between healthy and diseased pine and spruce are given. They show whole tracts of forests, individual trees and details from the tops of trees. It is shown that externally visible changes in a tree which would indicate environmental stress when it is grown in an ideal habitat, may, when present at the margin of its distribution area, merely be a reaction to naturally stressful conditions. The authors draw attention to the problems of recognizing and interpreting the symptoms of damage. One of the purposes of the trilingual text is to standardize the terms so that international cooperation in the investigation and documentation of forest damage may be simplified and misunderstandings eliminated.