San Xavier del Bac stands in the community of Wa:k within the Tohono O'odham Nation, south of Tucson. Jesuit missionary Eusebio Francisco Kino founded a mission there in 1692, but O'odham people already possessed their own language, agriculture, trade, religion, and relationships to the Sonoran Desert. A modest earlier church preceded the present building. Spanish missionization sought conversion and imperial control while also creating new economic and defensive arrangements. O'odham communities negotiated, adopted, resisted, and reshaped Catholic practices rather than passively receiving a complete European culture.

After the Spanish crown expelled Jesuits in 1767, Franciscans took responsibility for the mission. Construction of the present church began around 1783 under Franciscan direction and local patronage, with O'odham laborers and craftspeople forming fired brick, lime plaster, timber structures, and ornament. Completed substantially by 1797, the building combines a central dome, transept, twin-tower façade, choir loft, sacristy, and densely decorated sanctuary. One tower remains visually unfinished, a condition explained by construction history more reliably than popular legends about taxation or deliberate incompletion.

Interior sculpture, painted plaster, retablos, angels, saints, foliage, and architectural illusion combine late Baroque religious art with local materials and workmanship. The church supported Mass, sacraments, festivals, burial, teaching, farming, ranching, and mission administration. Mexican independence transferred the region from Spain to Mexico; the last resident Franciscan left in 1837, and the building endured intermittent care. The Gadsden Purchase brought the area into the United States in 1854, while the Diocese of Santa Fe and later Tucson restored regular clergy and school activity.

Earthquakes, water, heat, incompatible cement repairs, roof leaks, and tourism have threatened adobe and lime-based construction. Twentieth- and twenty-first-century campaigns led by the Patronato San Xavier have removed damaging materials, repaired masonry, conserved paintings and sculpture, and renewed exterior lime plaster using compatible methods. Tohono O'odham workers and trained conservators remain essential. The church retains exceptional eighteenth-century fabric and art, but continuing sacramental use and cyclical replastering mean surfaces are maintained rather than untouched. Modern lighting, safety systems, and visitor management are discreet additions.

The mission remains an active Roman Catholic parish serving the village of Wa:k and operates within the sovereign Tohono O'odham Nation, with diocesan and Franciscan relationships. The church is generally open without an admission fee outside services, ceremonies, conservation work, and community restrictions; donations support preservation. It is neither a state museum nor a Spanish-colonial relic detached from contemporary Indigenous life. Its significance joins O'odham endurance and agency, colonial mission history, living Catholic worship, exceptional Baroque art, and a conservation practice that treats traditional materials and community authority as inseparable.