Solar energy is the source of most of the living organisms on Earth so that the overall efficiency of oxygenic and/or non-oxygenic photosynthesis, when used to generate biomass, bioenergy and biofuels, is a critical point to be considered. This volume in the Advances in Photosynthesis and Respiration series, however, not only provides a comprehensive view of the current understanding of the photosynthetic mechanisms linked to bio-hydrogen production but also extends this view to the anaerobic-dark processes involved in transforming the solar-generated biomass into biogas along with a deep coverage of both structural and functional aspects of the main enzymes involved, such as nitrogenases and hydrogenases. The fifteen chapters of this book offer a broad coverage of this emerging research field and, it is hoped, will be accessible to most advanced undergraduates, graduate students, PhD students and researchers looking to broaden their knowledge on the photosynthetic and fermentation processes applied to hydrogen gas generation. For biologists, biochemists, biophysicists and microbiologists, this volume will provide a solid and quick starting base to get into biotechnological problems of “ microbial bioenergy”. This volume will also be of interest to advanced undergraduates in chemical engineering and biotechnology teachers wanting a single reference book on the latest understanding of the critical aspects of microbial bioenergy production. This volume is dedicated to both Hans Gaffron (1902-1979), who discovered H2-production by green algae under sulfur starvation in 1942, and Howard Gest (1921-2012), who first described H2-production by purple non-photosynthetic bacteria in 1949.