Biohydrogen can be generated by microorganisms such as microalgae and cyanobacteria through biophotolysis and catabolism of endogenous substrate. Biophotolysis occurs due to the effect of light on the microbial systems, resulting in dissociation of water into molecular hydrogen and oxygen. The light-dependent biophotolysis metabolic pathways can be differ­entiated into two distinct categories: direct photolysis and indirect photolysis. Whereas elec­trons derived from water lead to photosynthetic hydrogen production in biophotolysis, electrons from catabolism of endogenous substrate would result in hydrogen production in a distinct mechanism.