The assumption of Vertical Equilibrium (VE) and of parallel flow conditions, in general, is often applied to the modeling of how and displacement in natural porous media. However, the methodology for the development of the various models is rather intuitive, and no rigorous method is currently available. In this paper, we develop an asymptotic theory using as parameter the variable R(L) = L/H root k(V)/k(H). It is rigorously shown that the VE model is obtained as the leading order term of an asymptotic expansion with respect to 1/R(L)(2). Although this was numerically suspected, it is the first time that it is theoretically proved. Using this formulation, a series of special cases are subsequently obtained depending on the relative magnitude of gravity and capillary forces. In the absence of strong gravity effects, they generalize previous works by Zapata and Lake (1981), Yokoyama and Lake (1981) and lake and Hirasaki (1981), onimmiscible and miscible displacements. In the limit of gravity-segregated flow, we prove conditions for the fluids to be segregated and derive the Dupuit and Dietz (1953) approximations. Finally, we also discuss effects of capillarity and transverse dispersion.