The primary purpose of this article is to summarize current information on theoccurrence and distribution of MTBE in groundwater. Information is alsopresented on the sources and transport of MTBE to groundwater. Data from theU.S. Geological Survey's (USGS) National Water Quality Assessment (NAWQA) Program,as well as data from other sources, are summarized to describe: theoccurrence and distribution of low concentrations of MTBE in ambient groundwater;the effects of high concentrations of MTBE in groundwater used fordomestic and public supplies; the potential sources of MTBE; and, thelong-range transport of MTBE in groundwater. Includes 36 references, tables, figures.