The improvement of H-2 production capabilities of hydrogen (H-2)-producing microorganisms is a challenging issue. Microorganisms have evolved for fast growth and substrate utilization rather than H-2 production. To develop good H-2-producing biocatalysts, many studies have focused on the redirection and/or reconstruction of cellular metabolisms. These studies included the elimination of enzymes and carbon pathways interfering or competing with H-2 production, the incorporation of non-native metabolic pathways leading to H-2 production, the utilization of various carbon substrates, the rectification of H-2-producting enzymes (nitrogenase and hydrogenase) and photophosphorylation systems, and in silico pathway flux analysis, among others. Owing to these studies, significant improvements in the yield and rate of H-2 production, and in the stability of H-2 production activity, were reached. This review presents and discusses the recent developments in biohydrogen production, with a focus on metabolic pathway engineering. (C) 2011 Elsevier Ltd. All rights reserved.