El Cajon is a city in San Diego County, California, United States. In a valley surrounded by mountains, the city has acquired the nickname of "The Big Box." El Cajon is one of the region's premiere locations in which to live, work and play. As the fifth largest city in San Diego County, El Cajon boasts a diverse and vibrant community. With safe and attractive residential neighborhoods, parks, libraries and other amenities, award-winning schools, and a quality of life that is highly sought, El Cajon is a shining example of exceptional municipal service. El Cajon is a full service City, with first-class police and fire departments.