With all the current efforts to use non-fossil sources as a starting point for future energy solutions, consideration is also being given to using microbial activities as a direct or indirect source of energy production. This ranges from the use of algae as biomass or as H2 producers, anaerobic microorganisms to produce methane, hydrogen, and even electricity directly. This book deals with both theoretical and technical possibilities of using anaerobic microorganisms in combination with wastewater as a substrate source to produce biofuels and bioenergy in the form of biomass, CH4 and H2 as well as the corresponding power densities and electricity quantities in economically justifiable processes. Unique process facilities are widely addressed; however, special interest is also placed in biorefinery and circular economy related concepts. The theoretical background as well as application examples are presented.