This book is the result of data collected from Gas Research Institute's (GRI) program on the production of methane from various types of biomass and wastes, indicating, that the state of knowledge of this process at the fundamental level is deficient. This work documents the status of anaerobic digestion processes for conversion of biomass and wastes to methane at the commercial, developmental, and research levels and identifies the data gaps that exist between commercialization and development, process limiting steps, and research needs. The book also contains reviews on basic and applied research in the areas of microbiology and process development. It provides a basis for recommending research needs and priorities related to the improvement of yields, rates, process stability, net energy production, and process economics, with the emphasis on biomass rather than wastes.