Although it has been known for over a century that micro-organisms have the potential to reduce metals, more recent observations showing that a diversity of specialist bacteria and archaea can use such activities to conserve energy for growth under anaerobic conditions have opened up new and fascinating areas of research with potentially exciting practical applications (Lloyd, 2003). Micro-organisms have also evolved metal-resistance processes that often incorporate changes in the oxidation state of toxic metals. Several such resistance mechanisms, which do not support anaerobic growth, have been studied in detail by using the tools of molecular biology. Three obvious examples include resistance to Hg(II), As(V) and Ag(II) (Bruins et al., 2000). The molecular bases of respiratory metal-reduction processes have not, however, been studied in such fine detail, although rapid advances are expected in this area with the imminent availability of complete genome sequences for key metal-reducing bacteria, in combination with genomic, proteomic and metabolomic tools. This research is being driven forward both by the need to understand the fundamental basis of a range of biogeochemical cycles, and also by the possibility of harnessing such activities for a range of biotechnological applications. These include the bioremediation of metal-contaminated land and water (Lloyd & Lovley, 2001), the oxidation of xenobiotics under anaerobic conditions (Lovley & Anderson, 2000), metal recovery in combination with the formation of novel biocatalysts (Yong et al., 2002a) and even the generation of electricity from sediments (Bond et al., 2002).