The unicellular green alga Scenedesmus obliquus is capable of producing molecular hydrogen in the light. The prerequisite is the activation of the responsible enzyme hydrogenase by anaerobic adaptation. Photohydrogen production is coupled to the photosynthetic electron transport chain of green algae. Results of biochemical and molecular biological investigations of a purified NiFe hydrogenase and an cloned “Fe-only” hydrogenase are presented, and thus the existence of two hydrogenases is shown for Scenedesmus. The cyanobacterium Synechocystis sp. PCC 6803 is introduced as a genetic tool for future manipulations of S. obliquus hydrogenase. Construction of a simple pilot bioreactor for photohydrogen production is explained and the general use of light-dependent hydrogen production by algae as a future energy source is discussed.