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The freshwater-saltwater interface is a key feature in coastal aquifers. From a water supply perspective, this interface marks the seaward boundary of fresh groundwater resources, now known to extend tens of kilometers offshore in many world regions. From an oceanographic perspective, the freshwater-saltwater interface can be viewed instead as the landward boundary of subseafloor seawater-groundwater exchange, which exports nutrient-rich saline groundwater to the ocean in volumes similar to river discharge. The freshwater-saltwater interface thus ties together such seemingly diverse interests as water supply and nutrient cycling in continental shelf ecosystems. Still, the location of the freshwater-saltwater interface is very poorly known in all but the shallowest aquifers. Recently, a new generation of geophysical surveys has demonstrated the ability of CSEM and MT surveys to detect freshened groundwater below the seafloor at significant depths in four locations where large supplies of fresh groundwater offshore had already been identified. Integration between geophysical and hydrogeological approaches is still evolving.