Oxygenic photosynthetic microorganisms harness solar energy, converting it into chemical energy of carbohydrates by simultaneous splitting of water to molecular oxygen and protons. They can efficiently utilize CO, thus positively contributing to mitigation of CO emission from industrial activities. Under specific conditions, microalgae are also able to redirect electrons to production of hydrogen. Although the potential light conversion efficiency to hydrogen in microalgae is theoretically high, at about 10%, the system is restricted by biochemical and metabolic pathways. Indeed, there is much research and genetic engineering to be done before hydrogen produced by microalgae becomes an important component of future hydrogen economy. In this chapter, we mainly focus on hydrogen production by oxygenic photosynthetic microorganisms via the light-dependent direct and indirect biophotolysis pathways. © 2014 Elsevier B.V. All rights reserved.