Microbial fermentation is a potentially important member of a diverse portfolio of methods available for renewable production of hydrogen using biomass as a substrate. A major factor currently limiting commercial viability of fermentative hydrogen production is poor stoichiometric yield per mole of sugar and end-product feedback inhibition. Although several approaches have been tried to enhance microbial hydrogen productivity, genetic engineering technologies for most promising microorganisms are either unavailable or severely limited. Further efforts to overcome existing issues will need to rely on the ability to analyze, predict and engineer microbial metabolism in native H-2-producing strains as well as genetically engineerable strains with constructed H-2-metabolism. A shift in the type of biomass substrate from starch-based food crops to ligno-cellulosic crops and wastes that are economically and environmentally less costly to produce, yet more difficult to chemically and biologically process, presents technical challenges that are shared by all emerging biofuel technologies. A brief overview of the current status of fermentative H-2-production, including a discussion of directly and indirectly related emerging technologies, is presented. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.