Groundwater is considered the main source of water in many coastal areas. The increase of water demands increases the abstraction from aquifers which has resulted in lowering water tables and caused saltwater intrusion. Coastal aquifers lie within some of the most intensively exploited areas of the world. Saltwater intrusion is one of the main causes of groundwater quality degradation and a major challenge in the management of groundwater resources in coastal regions. Saltwater intrusion causes an increase of salt concentration in groundwater which places limitations on its uses. Excessive pumping always leads to a dramatic increase in saltwater intrusion. In coastal aquifers, the hydraulic gradient exists towards the sea which leads to flow of the excess freshwater to the sea. Seawater intrusion is a special category of groundwater contamination that threatens the health and possibly lives of the people living in coastal areas. The problems of saltwater intrusion into groundwater had become a considerable concern in many countries particularly in coastal areas. Seawater intrusion leads to the depletion of groundwater resources and should be prevented or controlled to protect water resources in coastal regions. The intrusion of saltwater in coastal aquifers has been investigated by several methods including geophysical methods, geochemical methods, experimental studies and mathematical models. This chapter presents a brief history of saltwater intrusion in coastal aquifers. The Nile Delta aquifer is one of the largest underground freshwater reservoirs in the world that attacked by saltwater intrusion. Large amounts of freshwater were damaged by salinization. Extensive studies were carried out to investigate saltwater intrusion in Nile Delta aquifer using numerical and field studies. Most of these investigations revealed that the seawater intrusion in the Nile Delta aquifer has extended to a distance of more than 100 km from the Mediterranean coast. The effect of climatic changes including the rise in the sea level has a significant effect on the position of the transition zone, and the groundwater quality would deteriorate in large areas of the Nile Delta aquifer.