Subsurface contamination due to immiscible organic liquids is a widespread problem which poses a serious threat to ground-water resources. In order to understand the movement of such materials in the subsurface, a mathematical model was developed for multiphase flow and multicomponent transport in porous media with water, NAPL and air or any subset of these phases. Numerical procedures for solving the system of coupled flow equations, based on various formulations of the governing equations, were compared. Accurate representation of three-phase permeability-saturation-capillary pressure (k-S-P) relations is crucial to model multiphase fluid movement and accurate models for interphase mass partitioning are critical to describe species transport. A detailed description of hysteresis in three-phase k-S-P relations was reported. Simplified models, which consider effects of nonwetting fluid entrapment, were shown to provide a reasonable compromise between accuracy, on the one hand, and efficiency and robustness, on the other. Laboratory studies of light and dense NAPLs in a 1 x 1.5 meter sand tank, involving measurements of water and NAPL pressures and saturations and component concentrations, are described. These studies were used to validate the mathematical model for multiphase flow and transport.