La Purisima Mission, one of 21 Spanish missions in California built as religious and military outposts by Spanish Catholics of the Franciscan Order between 1769 and 1823. Founded in 1787, this mission is now a California State Historic Park. Photo by Carol Highsmith.

The missions represented the first significant effort by Europeans to colonize the Pacific Coast region and "civilize" the indigenous Indian populations.

They gave Spain a valuable toehold in the frontier land. The settlers introduced European livestock, fruits, vegetables, cattle, horses and ranching -- but also diseases from which the native peoples had no immunity. After its successful revolution against Spain, Mexico shut down the missions in the 1830s.

Today, the missions along U.S. Highway 101 -- the old Mission Trail or El Camino Real (the Royal Road) are among California's oldest structures and its most-visited historic monuments.