The current understanding of the transformations of metals and metalloids by, and the effects of metals and metalloids on, microorganisms is reviewed with 200 references. Microbial transformations of several technologically prominent toxic metal systems, including arsenic, mercury, cadmium, lead and tin are discussed, in greater detail with respect to inorganic and organometallic ions. The implications of these transformations to the general problem of metal pollution, particularly in aquatic environments, are described. The effect of metal stress, plasmid transfer, and metal tolerance on bacteria is considered.