The following review article will serve to elucidate the existing state-of-the-art and breadth of technical understanding related to thermophilic aerobic biological wastewater treatment. The advantages of this technology include rapid biodegradation rates, low sludge yields, and excellent process stability. Substrate utilization rates reported in the technical literature are 3-10 times greater than that observed with analogous mesophilic processes, and sludge production rates are generally similar to anaerobic treatment processes. As such, thermophilic aerobic treatment has been used to biodegrade wastewaters from the pulp and paper industry, livestock production, and many other miscellaneous waste streams. Thermophilic aerobic processes are particularly advantageous for the treatment of high-strength wastewaters that can fully benefit from the rapid biodegradation rates and low sludge yields. High-strength wastewaters also contain the necessary energy content to facilitate autothermal operation such that exogenous heat input is not required. A theoretical energy balance is presented which predicts that COD removals of 20,000-40,000 mg l-1 coupled with an oxygen transfer efficiency of 10-20% are necessary for autoheating to thermophilic temperatures. Of the bacteria likely to proliferate in thermophilic aerobic bioreactors, relatively unique and specific nutritional requirements are common. In particular, thermophilic Bacillus spp. commonly exhibit a growth requirement for methionine. Most researchers have reported that thermophilic bacteria fail to aggregate, making biomass separation from the treated effluent a key design criterion. Further work on thermophilic aerobic treatment processes is also needed to identify optimum operating conditions, and determine the best method to accommodate the oxygen uptake rates of these systems.