(structure and concentration), their availability to the microorganisms, and the physicochemical characteristics of the environment. The capacity of a microbial population to degrade pollutants in an environmental matrix (soil, sediment, sludge, wastewater, etc.) can be enhanced either by stimulating the indigenous microorganisms by adding nutrients or electron acceptors (biostimulation) or by introducing specific competent microorganisms to the local population (bioaugmentation). Many biotic and abiotic factors make bioaugmentation a relatively unpredictable strategy of bioremediation. The addition of exogenous microorganisms may be justified for the treatment of recalcitrant compounds in sites devoid of significant populations of biodegrading microorganisms. Field practice shows that under optimal local environmental conditions the rate and extent, and, sometimes, the spectrum of pollutant degradation may be enhanced upon addition of an inoculant to remediate a chemical plume. However the most successful cases of bioaugmentation occur in confined systems, such as bioreactors in which the conditions can be controlled to favour survival and prolonged activity of the exogenous microbial population.