Energy is vital to global prosperity, yet dependence on fossil fuels as our primary energy source contributes to global climate change, environmental degradation, and health problems. Hydrogen offers tremendous potential as a clean, renewable energy currency. It has the highest gravimetric energy density of any known fuel and is compatible with electrochemical and combustion processes for energy conversion without the carbon emissions. This e-book summarizes the current state of the art and progress in biohydrogen production. Biohydrogen has significant economical since biological processes are much less energy intensive compared with electrolysis and thermo-chemical processes. Topics covered in this reference include progress in hydrogen production by light-driven processes, dark fermentation, hydrogen production from waste materials, the role of molecular engineering for enhanced hydrogen production, and post-production processing such as hydrogen purification and storage.