Biological treatment of odorous airstreams at wastewater treatment plants is being implemented more frequently in recent years largely because of relatively high costs of more traditional dry and wet chemical treatment methods. Performance of certain types of biological foul air treatment systems is improving to the point that we are more confident in recommending and implementing biological treatment in more situations today than ever before. Research and development work by Brown and Caldwell and others has helped increase the usefulness and performance of these systems.