The Sagrada Familia represents the high point of Gaudí's artistic expression. Throughout his career, Gaudí devoted himself to this work, especially in his later years, reaching the fullness of his naturalistic style. In this project, he achieved a masterful fusion between structural and ornamental elements, between the plastic and the aesthetic, between function and form, and between content and container. This effort resulted in a harmonious integration of all artistic disciplines in a logical and cohesive construction.

From 1915, Gaudí devoted himself predominantly to the Sagrada Familia, which encapsulates his architectural evolution. After completing the crypt and apse in a neo-Gothic style, he devised the rest of the temple in an organic form, emulating the shapes of nature, with prominent use of regular geometric forms. The interior was conceived to evoke a forest, with columns leaning like trees, arranged in a spiral, creating a simple and resistant structure. Gaudí applied here his previous learnings from works such as the Parc Güell and the crypt of the Colonia Güell, achieving a structurally sound and aesthetically harmonious church.

The Sagrada Familia has a Latin cross plan, with five central naves, a transept with three naves and an apse with seven chapels. It has three facades representing the Birth, Passion and Glory of Jesus, and at its completion, it will house eighteen towers: four on each portal (totaling twelve for the apostles), four on the transept (for the evangelists), one in the apse (dedicated to the Virgin) and the central tower-cimborium in honor of Jesus, which will rise to 172.5 meters in height. The church will be complemented by two sacristies next to the apse and three main chapels: that of the Assumption, in the apse, and those of Baptism and Penance near the main façade. In addition, a surrounding cloister will be used for processions and to protect the church from its surroundings. Gaudí imbued the Sagrada Familia with deep symbolism in its architecture and sculpture, assigning religious meanings to each component.

During Gaudí's lifetime, only the crypt, the apse and partially the Nativity façade, whose tower of San Bernabé he personally crowned, were completed. After his death, Domingo Sugrañes took the lead in the construction, and then several architects have continued his work, Jordi Faulí i Oller being the director of the works since 2012. The sculptural ornamentation involved artists such as Llorenç and Joan Matamala, Carles Mani, Jaume Busquets, Joaquim Ros i Bofarull, Etsuro Sotoo and Josep Maria Subirachs, the latter responsible for the decoration of the Passion façade.

In 2005, UNESCO included the Nativity façade and the crypt of the Sagrada Familia in the World Heritage Site "Works of Antoni Gaudí". The monument is listed in the registers of Cultural Assets of National Interest and Assets of Cultural Interest, being also one of the 12 Treasures of Spain since 2007. It was also chosen as one of the Seven Wonders of Catalonia in the same year. In 2010, Pope Benedict XVI designated it a minor basilica. In the same year, the newly built main nave received the City of Barcelona Award for Architecture and Urbanism. Popularly known as the "Cathedral of the poor", this name comes from the homonymous painting by the modernist painter Joaquín Mir.