Several hundred gravel bed, or subsurface flow (SF) wetland systems exist in the United States ranging in size from single family dwellings to municipal systems designed for flows up to 11,000 cu m/d. Most of these systems have been constructed in the period 1988 to 1992 without the benefit of a consensus on design, construction or operational procedures. The U.S. Environmental Protection Agency (EPA) commenced a continuing series of studies in 1989 to identify the critical issues and to develop appropriate process criteria for this concept. These efforts have included a detailed survey, on-site visits and performance evaluations, and special data collection and evaluation programs at selected sites. The focus of the paper is on the capability of these systems to remove biochemical oxygen demand (BOD5), total suspended solids (TSS), and ammonia nitrogen (NH3 as N), since these are the major water quality parameters controlled by the regulatory agencies in the U.S.