The 1986 amendments to the Safe Drinking Water Act (SDWA) declare granular activated carbon (GAC) to be the base-line technology against which all other technologies must be measured for synthetic organic chemical (SOC) control. The USEPA's Drinking Water Research Division (DWRD) has been active in determining the effectiveness of GAC in removing organic contaminants. Because GAC is typically employed in a fixed-bed reactor, treatment efficiency is best described by pilot or full-scale columns evaluated with the contaminated water in question. Because it is expensive and time consuming to conduct full-scale field studies, predictive computer modeling of GAC systems is an inherent part of the USEPA's Cost and Technology documents which support the SDWA. The article discusses the mathematical models used by the DWRD and the USEPA's Office of Drinking Water (ODW) to evaluate SOC control in natural waters by GAC treatment processes.