This book is based primarily on data and conclusions made from field studies. We have focused on undisturbed forest systems in an attempt to create a basic understanding of, and present basic mechanisms for the decomposition and transformation processes. Its emphasis is on boreal systems for the obvious reason that there appears to exist more data about these systems that could be synthesized. The information from temperate systems has largely supported and extended the conclusions, suggesting that the synthesis so far may be applicable to at least both types of systems.