This paper summarizes a five-year research effort on the development of a high-rate fluidized-bed biofilm system for remediation of chlorophenol contaminated groundwater. Laboratory-scale experiments with model compounds and actual contaminated groundwater included studies on aerobic and anoxic treatment, strategies for selective enrichment of chlorophenol degrading microorganisms, kinetics and stoichiometries of chlorophenol conversions, toxicity removal, growth and culture characterization, recovery from process upsets, and operation at suboptimal temperatures. The aerobic process using chlorophenols as the sole source of carbon and energy showed the greatest versatility and efficiency. The contaminated groundwater consisted of 2,3,4,6-tetrachlorophenol, 2,4,6-trichlorophenol and pentachlorophenol with total chlorophenol concentrations ranging from 45 to 55 mg/l. At room temperature, groundwater remediation resulted in over 99.9% mineralization at chlorophenol loading rates of 1000 mg/l/d and hydraulic retention times of less than 1h. At the groundwater temperature (7°C), similar removal efficiencies were obtained at chlorophenol loading rates of up to 740 mg/l/d. The laboratory results were successfully repeated in an on-site, pilot-scale demonstration.