For optimal tree care, a solid understanding of the biological fundamentals of trees is essential. Natural events, environmental changes, and human interventions such as pruning and damage affect the development of tree crowns. A fundamental and comprehensive understanding of the crown responses to these influences allows practitioners to understand the tree as a whole and not lose sight of it. Therefore, the aim of this manual is to illustrate the crown development of trees using important, common tree species as examples, to enable a vitality assessment based on crown characteristics, and to present selected aspects in more detail. In doing so, differences in the crown development of the tree species discussed are linked to their biological and ecological behavior. The basics, backgrounds, and methods for vitality assessment conveyed here.