This article reviews biological production processes of hydrogen as a renewable and alternative energy source. Biological hydrogen produced from renewable sources (biomass, water, and organic wastes) either biologically or photo-biologically is called "biohydrogen." The phenomenon of biological hydrogen production was observed one century ago. When the oil crisis broke out in 1970s, the technology started receiving attention, especially in hydrogen production by photosynthetic process. Biological hydrogen production is one of the alternative methods where processes can be operated at ambient temperatures and pressures, and are less energy intensive and more environmentally friendly. Biological production of hydrogen as a by-product of microorganism metabolism is an exciting new area of technology development that offers the potential production of usable hydrogen from a variety of renewable resources.