During 1990 and 1991 the U.S. Environmental Protection Agency (EPA) sponsored an effort to identify existing and planned constructed wetlands in the U.S., and to collect readily available information from operating systems. In addition to inquiries by telephone and mail, the effort included site visits to over 20 operating subsurface flow constructed wetlands. The inventory documented the presence of over 150 constructed wetland systems for wastewater treatment, including both free water surface (FWS) and subsurface flow (SF) systems. The majority of the systems identified were SF systems for treating municipal wastewater. FWS systems were separated into three groups based on the design level of effluent water quality. SF systems were separated into three groups based on the basic design approach. The inventory indicated that neither between nor within these groups was there consensus regarding basic hydraulic and engineering design criteria, system configuration, or any other aspect, such as type of vegetation, size and type of media, or pretreatment. Information on location, type of system, design approach, hydraulic and organic loading rates, costs, and other aspects is presented. Information gathered and "lessons learned" from the site visits are presented. Insufficient oxygen for nitrification appears to be a problem for both FWS and SF systems. Insufficient hydraulic design appears to be a problem for SF systems.