Traditionally only four deserts have been recognized in North America: the Great Basin, Mojave, Sonoran, and Chihuahuan. These deserts were recognized based on general aridity, soil types, and arid-adapted plants and animals. The San Joaquin Valley had been classified by many biologists as either a perennial or annual grassland, although some recognized the desert nature of the area in works peripheral to the specific nature of the area. After conducting field studies for 20-25 years on various plants and animals in many parts of the San Joaquin Valley, my colleagues and I set out to formally recognize the San Joaquin Desert that makes up about two thirds of the valley and adjacent areas. Based on coincident ranges of low precipitation, arid soils, and plants and animals that are clearly desert adapted, we delineated the boundaries of this fifth North American desert. Because of the high number of protected species, many of which are endemic to this desert, it is important to recognize the true nature of this part of California so that management actions to restore habitat do so with the aim of reestablishing the correct plant communities.