Dark fermentative bio-hydrogen production is not commercially exploited due to several factors hindering its production, making the process unfeasible on large scale. This study provided an in-depth and critical review of different factors of the dark fermentation process namely H-2-consumers and lactic acid bacteria in mixed microflora, light and heavy metal ions, furan derivatives and phenolic compounds, ammonia and H-2 concentrations and soluble metabolites viz. acetic acid, ethanol, propionic acid and butyric acid that may negatively affect H-2 production. For each of the inhibitors, the mechanism behind process inhibition was explained while strategies for reducing inhibition were outlined. Among the different inhibitors studied, furan derivatives and phenolic compounds suppressed biohydrogen production to a larger extent while the most common strategies reviewed for reducing inhibition included inoculum pre-treatment for suppressing H-2-consumers and dilution of reactor contents for decreasing inhibitor concentrations. Although these options are encouraging on small scale, the economic and technical feasibilities of implementing these strategies on larger scale require further investigation. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.