Editors' Summary: The major federal environmental regulatory schemes make frequent use of numerical standards. The consequences of exceeding these standards can be extremely serious—companies may be subjected to civil or even criminal liability, or required to undertake increased regulatory responsibilities. For example, liability under the Clean Water Act is based on exceedances of numerical limits in NPDES permits. The author of this Article asserts that the methods by which such numerical standards are developed and by which compliance with them is measured display intrinsic, irreducible imperfections, or variability. According to the author, these imperfections, unlessthey are accounted for, can result in false liability or excessive regulatory burdens. This Article explains what analytical variability is, and how it can affect the regulatory process. The author asserts that EPA is constitutionally required to account for analytical variability in its regulations, but that its attempts to do so thus far are inadequate. Finally, the author puts forward a series of recommendations to guide the regulated community in protecting itself from the adverse consequences of analytical variability.