The California Trail carried over 250,000 gold-seekers and farmers to the gold fields and rich farmlands of California during the 1840's and 1850's, the greatest mass migration in American history. Today, more than 1,000 miles of trail ruts and traces can still be seen in the vast undeveloped lands between Casper Wyoming and the West Coast, reminders of the sacrifices, struggles, and triumphs of early American travelers and settlers. More than 240 historic sites along the trail will eventually be available for public use and interpretation. The trail passes through the states of Missouri, Kansas, Nebraska, Colorado, Wyoming, Idaho, Utah, Nevada, Oregon, and California.

The Pony Express National Historic Trail was used by young men on fast paced horses to carry the nation's mail across the country, from St. Joseph, Missouri to Sacramento, California, in the unprecedented time of only ten days. Organized by private entrepreneurs, the horse-and-rider relay system became the nation's most direct and practical means of east-west communications before the telegraph. Though only in operation for 18 months, between April 1860 and October 1861, the trail proved the feasibility of a central overland transportation route, and played a vital role in aligning California with the Union in the years just before the Civil War. Most of the original trail has been obliterated either by time or human activities. Along many segments, the trail's actual route and exact length are matters of conjecture. However, approximately 120 historic sites may eventually be available to the public, including 50 existing Pony Express stations or station ruins.

Santa Fe emerged as the hub of the overland continental trade network linking Mexico and United States markets—a network that included not only the Old Spanish Trail, but also the Santa Fe Trail and El Camino Real de Tierra Adentro. After the United States took control of the Southwest in 1848 other routes to California emerged, and use of the Old Spanish Trail sharply declined. Because of its rich history and national significance, the Old Spanish Trail has been designated as a national historic trail.