The central theme of this book “ ” is focused on the biological machinery that microorganisms use to produce hydrogen gas. The book summarizes the achievements over the past decade in the biochemistry, structural and molecular biology, genomics and applied aspects of microbial H -production, including microbial fuel cells (MFC), by phototrophs such as purple sulfur and non-sulfur bacteria ( spp., and spp.) microalgae ( ) and cyanobacteria ( ) along with anaerobes and thermophiles such as . This is the first book of this series entirely devoted to microbial bio-hydrogen production and is intended to be a precious source of information for PhD students, researchers and undergraduates from disciplines such as microbiology, biochemistry, biotechnology, phohemistry and chemical engineering, interested in basic and applied sciences.