Numerical models are typically two or three dimensions in space. In two dimensions, flow occurs either in a horizontal plane or cross section (profile). Most numerical models applied to engineering and groundwater management problems are three dimensional. Boundaries are set at physical or hydraulic features and are represented mathematically as specified head, specified flow, and head-dependent conditions. Boundary conditions for a local scale model may be extracted from the solution of a regional scale model. Methods for implementing boundaries in numerical models, including the water-table boundary and seepage face are discussed.