The movement of nonaqueous phase organic liquids (NAPL) in a groundwater system involves both miscible and immiscible flow phenomena. A broad overview of the task of modeling these flows is presented along with a discussion of various problematic issues related to such a modeling effort. Descriptive equations are obtained through combination of the species balance equations for multiphase fluid flow in a porous medium and appropriate constitutive relationships. Tabulation and discussion of these constitutive relations yields insight into model requirements. Under certain assumptions, the modeling problem can be reduced to the solution of two nonlinear partial differential equations in two unknowns. An example simulation of the movement of NAPL emanating from two point sources in a confined aquifer is presented to illustrate present modeling capabilities.