Biogas as a technology and the factors that affect its productivity have both been well studied. Research has previously been done to look at the impact of temperature, pH, organic loading rate, carbon-to-nitrogen ratios, microbial populations and hydraulic retention time on different scales of biogas operations. Small scale biogas installations, of which many millions have been constructed and seem to be performing well, have been chosen as the area of focus for this paper. Such systems allow energy generation on site, thereby eliminating the need for energy intensive transport. This review paper explores the factors that govern the productivity and operational stability of biogas digesters, and goes on to highlight the most important factors for small scale operation. The review reveals that the key factors that govern the productivity and stability of small scale systems are feedstock variability, retention time, temperature and acidity of the system. These are the main parameters which should, therefore, be considered when looking into the potential of biogas as a widespread technology to be deployed to meet energy demands. (C) 2014 Elsevier Ltd. All rights reserved.