Slope stability analyses carried out in terms of effective stress require an understanding of the distribution of pore-water pressures in a slope. This understanding must be based on a knowledge of the groundwater flow system, which is in turn dependent on the regional geologic environment and the configuration of hydraulic conductivity contrasts. This paper presents several computer simulations of flow systems in a variety of hypothetical slopes. Results show that the presence of low-conductivity units at the surface or at depth can be extremely detrimental to stability, particularly if they confine units of higher conductivity. The contrast in conductivity need not be more than two orders of magnitude. Such situations are common in thrust blocks, interbedded sedimentary rocks, weathering profiles, and deformed metamorphic rocks. Groundwater conditions critical to stability arise in anisotropic formations, where the axis of maximum conductivity is parallel to the dip of the slope. Fluctuations in regional flow systems can be critical to the stability of unconsolidated, terraced sediments.