In many coastal areas around the globe groundwater constitutes the main, if not the only, sources of fresh water. This is typically true in arid and semi-arid regions where rainfall is scarce and surface water resources are almost absent. Due to their milder climatic conditions as compared to inland areas, urbanization and development activities are mainly encountered in the coastal areas imposing an additional stress on the already exhausted aquifers. Adjacent saltwater water bodies encourage such aquifers causing considerable degradation in the water quality. This paper presents an overview of the saltwater intrusion phenomenon into coastal aquifers. The approaches of simulation and the mechanism of intrusion are elaborated with more focus on the dispersion zone approach. The initial and boundary conditions for the problem are discussed. The problem is very sensitive to the seaside boundary through which the saltwater intrudes into the aquifer. A study case for the Nile Delta aquifer in Egypt is presented and results are compared with field data.