Franciscan priests established 21 missions between 1769 and 1833 in Alta California, accompanied by military outposts. Their goal was to spread Christianity among the local Native Americans, as well as to affirm Spanish, and later Mexican, claims to the region.

To facilitate overland travel, the mission settlements were situated approximately 30 miles (48 kilometers) apart, about one day's journey on horseback, or three days on foot. The entire trail eventually became a 600-mile (966-kilometer) long "California Mission Trail." Rev. Lasuén successfully argued that filling in the empty spaces along El Camino Real with additional outposts would provide much-needed rest stops where travelers could take lodging in relative safety and comfort.^{[1]: 132 [2]: 152} Heavy freight movement was practical only via water. Tradition has it that the padres sprinkled mustard seeds along the trail to mark it with bright yellow flowers.^{[3]: 79 [4]: 260}

Today a growing number of people, calling themselves California Mission Walkers, hike the mission trail route, usually in segments between the missions.^[5] Walking the trail is a way to connect with the history of the missions. For some it represents a spiritual pilgrimage, inspired by Jesuit priest Richard Roos' 1985 book, Christwalk.^[6] The loosely organized group is attempting to formalize the route and establish markers, similar to the 330-mile (530 km) El Camino de Santiago, in Spain.

There were 21 missions, 8 asistencias, and 5 estancias in what is now California.

Two Franciscan missions, Mission Puerto de Purísima Concepción and Mission San Pedro y San Pablo de Bicuñer, were constructed within the present-day borders of California but were administered as part of the Spanish missions of Pimería Alta. As such, they are not considered a part of the 21 missions of Alta California.

• Bibliography of California history

On Spanish Missions:

• Spanish missions in Arizona
• Spanish missions in Baja California
• Spanish missions in the Sonoran Desert (including Sonora and southern Arizona)

On California history:

• Juan Bautista de Anza National Historic Trail
• History of California through 1899
• History of the west coast of North America
• Mission Vieja

On general missionary history:

• Catholic Church and the Age of Discovery
• History of Christian Missions
• List of the oldest churches in Mexico
• Missionary

On colonial Spanish American history:

• Spanish colonization of the Americas
• California mission clash of cultures
• Indian Reductions
• California Genocide
• Native Americans in the United States