The ability of microbial cells to sequester solutes selectively from aquatic solutions, via nonmetabolically mediated pathways, has been termed biosorption. The mechanism of biosorption has been shown not to be simple and often specific to the biomass-solute pair. The understanding of the mechanism at play, in each biosorption system, is a prerequisite for the understanding of the stoichiometry, the equilibrium, the kinetics, the selectivity, and the engineering process application potential. Biosorption has been studied mostly for inorganic ionic solutes, but there is also reported work on the biosorption of organic molecules. Reference is also made to the biosorption engineering application issues.