Mission Viejo (; corruption of Misión Vieja, Spanish for "Old Mission") is a commuter city located within Orange County, California, United States, in the Saddleback Valley. Mission Viejo is considered one of the largest master-planned communities ever built under a single project in the United States and is rivaled only by Highlands Ranch, Colorado in size. Its population as of 2019 was estimated at 94,381.Mission Viejo is suburban in nature and culture, and consists of residential properties, offices and businesses. The city is noted for its tree-lined neighborhoods, receiving recognition from the National Arbor Day Foundation. The city's name is a reference to Rancho Mission Viejo, a large Spanish land grant from which the community was founded.