Bioremediation of organic pollutants and heavy metals by use of microorganisms represents a safe, inexpensive, and environmentally-friendly concept in modern environmental engineering. During the last three decades intense efforts have been made by microbiologists and environmental engineers in the isolation and characterization of microorganisms capable of degradation, transformation and detoxification of recalcitrant chemical compounds of environmental concern: (polyhalogenated) dibenzo-p-dioxins, dibenzofurans, and diphenyl ethers. Special emphasis is placed on the potential of molecular biology techniques to improve presently available biocatalysts.