The large diversity of metabolic capacities and the high genetic plasticity of microorganisms allow them to degrade virtually all organic compounds of natural or anthropogenic (xenobiotics) origin including those that are sources of environmental pollution. Thus microorganisms are major actors to eliminate or alleviate pollutions in the environment. The natural attenuation processes due to microbial activities (biodegradation and/or biotransformation) as well as the possibilities of using microorganisms in preventive treatments and bioremediation – biostimulation, bioaugmentation, rhizostimulation, bioleaching, and bioimmobilization – are presented. The main methods for microbial treatment of pollution, the chemical structure and the origin of the major pollutants, as well as the mechanisms of degradation by microorganisms – on the basis of physiological, biochemical, and genetic approaches – are described. Examples of treatments are presented for urban wastewater (activated sludge, lagoons, and planted beds), solid wastes (aerobic treatment or composting, anaerobic treatment and methanization, discharges), gaseous effluents, pesticides, polychlorobiphenyls, and finally hydrocarbons and petroleum products in the marine environment.