Bioelectrochemical systems rely upon bioelectrocatalysts of microbial as well as enzymatic nature. Both bioelectrocatalysts serve the same purpose of enabling or accelerating a given electrochemical reaction but they often differ significantly in their properties as well as degree of scientific understanding. This review characterizes and compares the advantages and disadvantages of both classes of bioelectrocatalysts - discussed on three selected bioelectrochemical reactions: (i) the anodic glucose oxidation, (ii) the cathodic oxygen reduction and (iii) anodic conversion of light to electricity. The article will not only focus in the respective bioelectrocatalyst application but will also highlight the current level of understanding and address future challenges and research needs. (C) 2012 Elsevier Ltd. All rights reserved.