The increasing demand for electrical energy and environmental concerns associated with conventional means of its generation drive the interest in alternative fuels. Biohydrogen, widely considered as fuel of the future, is one of such alternatives. To date, research results suggest that biological routes are the most promising for hydrogen production, especially dark (hydrogen) fermentation. Hydrogen fermentation can be performed with agricultural and food processing wastes as substrates. In this paper the most important factors influencing dark fermentation are reviewed and analyzed. These are: pH, partial pressure, temperature, and retention time. The biohydrogen generation efficiency is also presented with respect to different substrates. It should be also pointed out that many factors are still unknown; thus, the process requires conducting further research.