The review deals with photosynthetic H2 production by various organisms, paying a speciál attention to eukaryotes, It includes a generál characterization of the catalyzing enzymes (hydrogenase and nitrogenase), quantum efficiency, the kinetics and mechanism of H2 photoevolution, the distribution and activity of H2 photoproducers (33 genera of eukaryotes), physiological functions of this process as well as recent developments in photobiological hydrogen technology.