We argue that there is a need for a more precise of PFAS in a way that avoids including compounds with single CF 3 -, -CF 2 -, or &z.dbdsl;CF- groups and excludes TFA and compounds that degrade to just give TFA. An example that meets this need is the definition by the U.S. Environmental Protection Agency of PFAS as "per- and polyfluorinated substances that structurally contain the unit R-(CF 2 )-C(F)(R 1 )R 2 . Both the CF 2 and CF moieties are saturated carbons and none of the R groups (R, R 1 , or R 2 ) can be hydrogen". Adoption of this definition, or one like it, would place future technical and regulatory discussions of the environmental impacts of organo-fluorine compounds on a sounder technical footing by focusing PFAS discussions and regulation on long-chain perfluoroalkyl sulfonic acids and perfluoroalkyl carboxylic acids. Many existing definitions of PFAS are overly broad, there is a strong case for a more precise definition of regulated PFAS.