Groundwater flow in the saturated zone is governed by the differential equation [Eq. (2.100)] with its boundary and initial conditions that we focused on in Chap. 2. The problem is generally three-dimensional but the most common type is a planar groundwater flow (seepage under or through a dam construction and, so on). Another example is a potential planar groundwater flow towards a system of wells. We simplify both cases neglecting the 3-D character of a flow. Numerical methods can solve even 3-D problems without software and hardware difficulties, though there are some requirements connected with the use of a 3-D model (e.g. setting the coefficients of hydraulic conductivity in a vertical and a horizontal direction in every layer of the model, a bigger precision of measurement of the inflow of water to pumping wells, knowledge of the position of every layer). It is therefore advisable to use 3-D models of groundwater flow only when you have a good geological survey to gather sufficient data. There are problems that directly require the use of a 3-D model such as greate thickness of the aquifer, partially penetrated wells and partial boundary conditions.