Rotating biological contactors (RBCs) are relatively new to secondary wastewater treatment in the United States, with over 500 municipal facilities being installed in the last decade. In addition to secondary treatment applications, RBCs have also been used successfully to upgrade marginal treatment facilities and to provide nitrification. Because of a variety of problems related to the design, construction, operation, and application of RBC facilities throughout the country, the U.S. Environmental Protection Agency has undertaken a number of research projects to investigate and identify the nature and extent of these problems and to determine possible solutions. These efforts have indicated that when properly designed, built, and operated, RBCs can provide an acceptable alternative biological treatment system.