The Old San Antonio Road, popularly known as El Camino Real, was one of Texas’s oldest and most significant trans-Texas routes, but rather than a single fixed road it was a changing network of Indian trails, natural stream crossings, and exploration routes that served commerce, migration, and communication during the Spanish colonial period. Its courses shifted with weather, Indian relations, terrain, transportation, settlement, and military needs, and although many later identified the Camino de Arriba route, which became State Highway 21, as especially important, it was only one fragment of a much larger system. The first known recorded trans-Texas journey along this network came in 1691, when the Teran-Mazanet expedition crossed the Rio Grande and followed the Camino de los Tejas into far East Texas, while other segments appeared later, including the portion connecting Bastrop and Crockett after 1790 and another between Nacogdoches and Alto about 1830. Some routes likely traced much older Indian paths, and the gradual southeastward shift of the Presidio del Rio Grande Road in the eighteenth and nineteenth centuries may have reflected Apache and Comanche threats to Spanish travelers. These caminos reales were among the most significant factors in the conquest and colonization of pre-Republic Texas, spurred in part by French ambitions such as Rene Robert Cavalier, Sieur de La Salle’s 1685 attempt to colonize Texas, which alarmed Spanish authorities and prompted military expeditions north across the Rio Grande. Communities arose as way stations between these outposts and Presidio del Rio Grande, with San Antonio de Bexar becoming the first and the hub of the regional roads. Until 1821, Spanish officials sought to strengthen Texas by encouraging civilian settlement between Bexar and Nacogdoches, and as frontier settlements spread, trails shifted to serve them. The road intended for defense also became a conduit for resistance to Spanish rule, and the evolving network influenced the strategy of the Texas Revolution. After the Republic of Texas was established, immigrants from the U.S. used these routes, including the camino real, and after the Mexican-American War the Camino de Arriba regained prominence by guiding Gold Rush travelers from East Texas to San Antonio. During the Civil War it again became important for moving cotton from East Texas to San Antonio and Laredo and for transporting troops and supplies from San Antonio, Bastrop, Crockett, and Nacogdoches to Louisiana. After the Civil War and economic decline, large segments of the trans-Texas Spanish roads were abandoned for shorter routes to new markets, and the remaining portions mostly served local functions. The Old San Antonio Road’s enduring significance lay in its role within the oldest transportation, communication, and military road network of the Spanish colony, the Republic of Texas, and later the state, serving as a post road decades before the Republic developed its own post roads, as a cattle trail long before the Chisholm Trail, as military roads linking forts on New Spain’s northern frontier, and ultimately as a principal avenue of immigration for Anglo-Americans who helped shape Texas.