In recent years aerobic digestion of sludge has become an increasingly popular method of stabilizing sludges prior to ultimate disposal. Many of the design requirements, however, have been based on laboratory studies and many of the parameters that affect operation and performance of aerobic digesters have not been studied in detail. As a result, specific design requirements that would insure consistently good performance from all aerobic digesters have not been developed. Formerly, the Technical Investigation Branch of the Surveillance and Analysis Division, and presently, the Operation and Maintenance Section of the Air and Water Division, provides as one of its functions technical assistance concerning the operation and maintenance of wastewater treatment facilities. This assistance normally is in the form of on-site operator training and plant evaluation. Another specific objective of this program is to provide 'feedback' concerning various design and training deficiencies based on actual operational experiences and observations. The following data on aerobic digesters represents the result of such experience plus a summarization of other pertinent design data.