Biological hydrogen production processes offer a technique through which renewable energy sources like biomass can be utilized for the generation of the cleanest energy carrier for the use of mankind. Hydrogen intensive research work has already been carried out on the advancement of these processes, such as the development of genetically modified microorganism, metabolic engineering, improvement of the reactor designs, use of different solid matrices for the immobilization of whole cells, biochemical assisted bioreactor, development of two-stage processes, etc. for higher H(2)-production rates. Maximum H(2) yield is found to be 7.1 mol H(2)/mol glucose. However, major bottlenecks for the commercialization of these processes are lower H(2) yield and rate of H(2) production. Suitable microbial cultures are required to handle waste materials efficiently, which are usually complex in nature. This will serve dual purposes: clean energy generation and bioremediation. Scale-up studies on fermentative H(2)-production processes have been done successfully. Pilot plant trials of the photo-fermentation processes require more attention. Use of cheaper raw materials and efficient biological hydrogen production processes will surely make them more competitive with the conventional H(2) generation processes in near future. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.