The paper presents the results of a review and investigation of the activated biofilter (ABF) and activated biofilter/activated sludge (ABF/AS) technologies and a review of operating records of several municipal plants in the U.S. using these technologies. The overall objective of the study was to understand and evaluate the background and performance of these technologies. To meet that objective, a series of steps were planned and executed. These included a visit to the ABF system manufacturer to obtain copies of the patent, design brochures and articles, list of installations, and operational data from several facilities. After reviewing this information, 43 facilities, each treating domestic wastewater and having a minimum design capacity of 3790 cu m/day (1 mgd), were contacted to obtain general information such as other unit processes at the facility, performance and age of the system, and operator satisfaction with the process.