The ability of microalgae to produce hydrogen was first reported by Gaffron and Rubin in 1942 (Gaffron and Rubin, 1942). However, the observed emission of hydrogen was transient and the amount was very minimal. In the late 1990s, Melis and co-workers demon­strated that sulfur deprivation changes cellular metabolism and allows algal culture to switch from aerobic photosynthetic growth to an anaerobic physiology state. Switching to anaerobic condition allows microalgal cultures to generate significant amounts of hydrogen for an extended period of time (Melis et al., 2000). This major breakthrough makes sustainable hydrogen production in a microalgal system a possibility. Over the years, extensive studies have been done to understand the physiology and metabolic adaptation resulting from sulfur depletion for better manipulation of biohydrogen production. Here our current understanding of hydrogen production in microalgae is highlighted and possible metabolic engineering/biotechnology strategies for improving hydrogen production are discussed.