Fossil fuels are becoming depleted and their excessive utilization is bringing about destructive climate changes on a global scale. Hydrogen is the cleanest energy carrier known today. This article reviews the biochemistry and enzymology of various biological hydrogen production technologies. Direct biophotolysis is the most elegant and most promising approach, but it is currently also the most expensive one. Photofermentation and dark fermentation technologies are closer to large-scale application. The benefits and drawbacks of the various approaches are discussed, and important directions for future research and development are highlighted.