Facultative anaerobes play important roles in H-2 production by biological routes. In the present study, a comprehensive insight on the importance of formate lyase in H-2 production by facultative microorganisms has been focused. Structural details of formate hydrogen lyase reveal the involvement of two enzymes, formate dehydrogenase H (FdhH) and hydrogenase (Hyd) along with electron carriers. The monocistronic fdhF gene codes for the functional formate dehydrogenase H. Functionality of these complexes are dependent on maturation system governed by FDH-D and FDH-E. The overall transcription of FHL complex is governed by protein encoded by fhlA gene. Under anaerobic condition, the pyruvate gets cleave into acetyl-CoA and formate by pyruvate-formate-lyase (PFL). The transporter proteins FocA and FocB helps in transporting formate. During dark fermentation, the physico-chemical parameters such as pH, HRTs, partial pressure play a crucial role in influencing formate degradation to molecular H-2. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.