The pore-water pressures in the dike body and subsoil constitute a significant part of the load on the dike. Consequently, insight into the groundwater flow pattern, characterized by the piezometric head and the pore-water velocities as a function of time and place, is required. The groundwater flow pattern can be determined in the following ways from: analytical and numerical computer models; analogue models; with the aid of graphical methods; and pore-water pressure measurements and piezometer observations, that is, from observations in the field. This chapter deals with these matters in detail. A number of aspects of the groundwater flow are dealt with in more detail in the following, namely: steady and unsteady flow; steady flow for isotropic and anisotropic permeability; and hydraulic fracturing and limit potential head. Observations of piezometric head in the sandy layers with piezometers and pore-water pressure measurements in poorly permeable soil layers provide important information for the gaining of insight into the groundwater flow at dikes.