The importance of magnesium in the element cycling of forest ecosystems has only been fully recognized since the widespread symptoms of forest magnesium deficiency were identified to be a major component in the damage phenomena. The intensive research during the last 15 years revealed not only details on the role of magnesium as a nutrient in forest ecosystems, but led also to a more comprehensive understanding of these ecosystems themselves and their functioning. This book tries to summarize the knowledge of magnesium and its role in forest ecosystems gathered from recent studies as well as from historical experiences and findings. The results of these studies in most cases point to the activity of man and his influence on the ecosphere. Therefore, when we understand the processes and effects described in this book, perhaps we can learn not only about a single nutrient like magnesium and forest ecosystems but also about ourselves.