This chapter reviews what is known about the interactions of microorganisms with key radionuclides, and where appropriate to discuss how such interactions can impact on the mobility of radionuclides in the environment. The role of microbial products in promoting colloid formation is an interesting, but generally poorly studied area, especially in the context of radionuclide migration. In addition, microbial activity will have a profound effect on the solubility of radionuclides via the following mechanisms: biosorption, bioaccumulation, biotransformations, biomineralization, and microbially-enhanced chemisorption of heavy metals. A potentially economical alternative is to develop biotechnological approaches based on natural biogeochemical transformations, that could be used in the sediment or soil to either extract the metals or stabilize them in forms that are immobile or non-toxic. Oxidation state is the most important factor controlling the speciation and therefore the environmental behavior of the actinides. The term biosorption is used to describe the metabolism-independent sorption of heavy metals and radionuclides to biomass.