These articles discuss aerobic and anaerobic biological degradation for dehalogenating sites contaminated with pesticides and chlorinated solvents. Bench-and field-scale studies are described, as are microcosm studies, numerical simulations, and site characteristics and their effect on the stability of methanotrophic community. Methods discussed include air venting, alternative electron donors, biofilm reactors, surfactants, municipal digester sludge, iron enhancement, and sulfate reduction to improve conditions for microbial consortia.