Trees are living organisms that are subject to attack by decay or disease-causing natural enemies in the course of their life span. When they suffer from these intrusions, they often express visible symptoms which may be structural or physiological. Tree managers can learn to recognise such symptoms and match them to different kinds of causes. The technique, known as visual tree assessment (VTA), identifies the specific symptoms which can be regarded as the body language of trees.

Even though trees cannot tell us about their ailments, they can communicate with us by displaying a wide range of visual hints, which can assist diagnosis from which treatments could be prescribed to help them. Timely treatment can help trees to lessen and contain the harmful impacts, to recover and regain health, and to render them stable and safe. They are important measures to reduce the risk of trees to life and property especially in cities.