Concerns over groundwater contamination by inorganic and organic chemicals in the subsurface, due to surface pills or leaks from underground storage facilities, have created a great impetus for the development of computational method of the transport process in the unsaturated (vadose) zone. Such methods are based on a conceptual framework, on subsequent theoretical developments, and, ultimately, on particular models adapted to the problem at hand. The purpose of these developments is twofold: to improve the basic understanding of the transport process, and to provide quantitative, predictive tools. In turn, these tools will be used to predict the future spread of pollutants in near-surface geologic environments, and for management purposes, i.e., to minimize the hazard of polluting underlying groundwater supplies. Our interest lies to a much larger extent in transport occurring at the field (formation) scale than at the laboratory (core) scale. This chapter focuses on some recent developments in the theory and in models of vadose-zone transport at the field scale. One of the distinctive features of a porous medium at this scale is the spatial heterogeneity of its properties which affect transport. This spatial heterogeneity is generally of an irregular fashion and occurs on a scale which is not captured by laboratory samples.