The salinization of groundwater may occur as a result of evaporite dissolution, irrigation, and the associated evapotranspiration, and contamination by seawater. When salts are dissolved, the groundwater density is increased, which influences flow and transport processes. The piezometric heads of saltwater correspond to higher pressures than their freshwater equivalents. Density dependent groundwater flow and its occurrence in coastal seawater intrusion problems are summarized. The Boussinesq approximation and the usual conceptual assumptions for seawater intrusion calculations are also presented. The concept of a sharp interface between seawater and freshwater is discussed.