Introduction Background Necessity of CO2-Neutral Energy Sources Energizing the Future with Alternative Nonconventional/Renewable Energy Sources Role of Biomass and Biotechnological Processes for Renewable Energy Production Biofuels from Organic Wastes Comparative Studies with the Conventional Process Biohythane: Potential as a Future Fuel Conclusion References Microbiology of the Biohythane Production Process Introduction Hydrogen Production by Dark Fermentation Microbial Basis of Methanogenesis Microbial Interactions Conclusions References Biochemistry of the Biohythane Production Process Introduction Biochemistry behind Dark-Fermentative Hydrogen Production Biochemistry behind Biomethane Production Challenges and Opportunities toward Commercialization of Biohythane Technology Conclusions References Mathematical Modeling and Simulation of Biohydrogen Production Processes Introduction Development of Mathematical Models to Correlate Substrate and Biomass Concentration with Time Substrate Inhibition Model Determination of Cell Growth Kinetic Parameters Cumulative Hydrogen Production by the Modified Gompertz Equation Development of Mathematical Models for Cell Growth Kinetics in a Packed-Bed Reactor Conclusion References Modeling and Simulation of the Biomethanation Process Using Organic Wastes Introduction Microbiology Biochemistry Thermodynamics and Kinetics.