Seawater intrusion represents the greatest water quality threat since the early 1900s to Orange County’s groundwater basin that supplies 75 percent of the water demands of 2.5 million people and businesses in north-central Orange County. The Orange County Water District (OCWD) manages the basin and has employed various methods to combat this threat. OCWD purchased/recharged large volumes of Colorado River water in the 1950s/1960s to try to fill the basin and reduce the inland gradient from the ocean. Seawater intrusion barriers, consisting of a series of freshwater injection wells, were constructed at the Alamitos Gap (Long Beach/Seal Beach) and Talbert Gap (Fountain Valley/Huntington Beach), in 1965 and 1976, respectively. They have helped protect the basin from seawater intrusion, but both barriers required expansions in recent past years after investigations revealed seawater was migrating inland. Folding, faulting, and erosional unconformities have challenged groundwater scientists attempting to accurately interpret and model the complex groundwater flow regime along the coast. After initial hints of a problem, the loss, in 2012, of a City of Huntington Beach production well due to seawater intrusion caught OCWD by surprise because it occurred in an area known as the Sunset Gap where it was unexpected and no monitoring or production wells existed. Two geophysical surveys, seven multi-depth monitoring wells, and 100s of hours of data analysis later, OCWD has a much better understanding of the likely sources and extent of the seawater intrusion in an area covering ~4 square miles beneath the Naval Weapons Station Seal Beach. Measures are clearly needed to contain/prevent further movement of intruded sea water. To that end, a groundwater model is being developed to help OCWD evaluate different extraction/injection well configurations. Much work remains, but a third seawater barrier may someday become a reality to protect Orange County’s vital groundwater resource.