Metabolic engineering is one of the promising areas which can be advanta­geously used to enhance H2 production rate in dark fermentation processes. By the use of recombinant DNA technology one can try to restructure metabolic network to improve the production of H2. Microbial metabolic manipulation by gene over expression, mutation and gene knocking out techniques were used for this purpose. H2 molar yields can be increased significantly through metabolic engi­neering efforts [109]. Table 6 documents some of the work carried out in this area pertaining to fermentative H2 production. By engineering the genetic expression of microorganisms the H2 production rate can be influenced directly or indirectly.