Methyl tertiary butyl ether, or MTBE, is a volatile solvent that had been widely used as an additive to elevate the octane level of gasoline. As a result of concerns regarding the presence of MTBE in groundwater, the use of MTBE as a gasoline oxygenate has largely disappeared. Nonetheless, there are thousands of sites where questions regarding the historical exposure of citizens remain. ChemRisk^® has performed many risk assessments, delivered many scientific presentations regarding the levels of exposure, and have published a number of papers on this topic in the peer reviewed literature.