Dark fermentative hydrogen production is an attractive route to the renewable production of hydrogen for a number of reasons. At least in its initial employment, it would use readily available waste streams as substrate. The required reactors would probably be relatively simple in design and based on technology that is already well known and widely used. The metabolic pathways involved are well understood and are reviewed here. A large amount of research has focused on factors affecting hydrogen yields during fermentation of various pure and waste substrates by either defined bacterial cultures or natural microbial flora and some of the pertinent highlights are discussed. Finally, known fermentation pathways can deliver at most 4H(2)/glucose, at best a 33% yield. Four different strategies to extract more hydrogen or energy have been proposed and are currently being investigated. The current progress in this direction is presented. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.