This book is published as part of the broad area of research on the changing global climate and its impact on the environment. After an introduction into the complexities of the subject, the history and present state of timberline research are outlined. Chapters on the tree species at timberline and on the relationship of timberline elevation to marcroclimate, climate character and the mass-elevation effect follow. The main chapter deals with the physiognomic and ecological differentiation of altitudinal timberlines, in particular with the timberline controlling physical and biological factors, their interactions and their influence on the spatial structures and temporal dynamics in the timberline ecotone. Also, the feedbacks of trees and trees stands on the timberline environment are considered. This is the base for understanding the response of timberlines to climatically driven changes, which are considered in the last chapters.