Microbial cell – soluble species interactions can be part of technologies for the treatment of metal/metalloid and radionuclide bearing water streams in order to sequester the targeted species. Interactions of microbial cells and soluble targeted species include passive and active processes of metabolically inactive or active biomass, and result in the reduction of their mobility and toxicity. Different parts of the cell may sequester targeted species via processes such as complexation, chelation, coordination, ion exchange, precipitation and reduction. Collectively, these mechanisms have been referred to as sorption and the overall phenomenon as biosorption. The term biosorption is generally used to describe the passive interaction of microbial biomass with targeted species. The technologies based on these processes, lead to the set up of units, mainly in the form of packed bed reactors similar to the configuration of ion exchange resins reactors, placed at the end of a treatment process as a polishing stage. In order to maintain durability of the sorbent, the microbial cells harvested from different sources, are formulated into particles by way of immobilization – pelletization. In the early years of Biosorption, a significant effort was devoted to study the reusability of the sorbent by repeated sorption – desorption cycles, in order to reduce the operating cost of the technology. The availability of the biosorbent material, the reversibility of the desorption process, the presence of competing co-ions and organic molecules, posed significant scepticism and finally serious doubt about the industrial applicability of biosorption as a stand alone technology. However the mechanisms are active and present in biological reactors, and can contribute to overall species sequestering. Biological reactors based on active microbial biomass as alternative to passive sorption, exploit the self regenerating features of living biomass along with the traits of microbial metabolism. Active cells produce metabolites (i.e. EPS, simple inorganic moieties etc.) interacting chemically with the targeted species. The active biomass offers the additional attractive feature of forming biofilms on the surface of carrier materials allowing a natural way of cell immobilization. Different biofilm reactor configurations e.g. static or moving bed filters, fluidized bed reactors, rotating biological contactors support the development of biofilms. Conditions such as temperature, pH, presence of toxic compounds etc. should be considered in the applicability of the technology. Important metabolically mediated immobilization processes for metal/metalloid and radionuclide species are bioprecipitation and bioreduction. Bioprecipitation processes include the transformation of soluble species to insoluble hydroxides, carbonates, phosphates, sulfides or metal – organic complexes as a result of the microbial metabolism. In the case of biological reduction, the cells may use the species as terminal electron acceptors in anoxic environments to produce energy or reduce the toxicity of the cells microenvironment. Such processes form the basis for treatment technologies which are recently developed and applied both in pilot and full scale.