La Sagrada Familia is one of Gaudí's most famous works in Barcelona. It's a giant Basilica that has been under construction since 1882 (that's not a typing error!) and it's not expected to be completed for between 30 to 80 years ( time for completion varies depending on different sources of information).
Much controversy surrounds the building of the Sagrada Familia. Today new construction materials are being used which, some feel, Gaudí himself would not have used.
Gaudí played an active role in directing the construction of the Sagrada Familia until his death in 1926. He would often request that work be modified and adjusted until it was exactly what he had in mind. However today, because of the nature of the existing designs, his work is partly open to interpretation.
Interpretation of the ordinal designs by present day architects is particularly challenging because the actual construction stones are irregularly shaped.
The building is still under construction so be prepared to see a lot of work continuing when you visit. However this in itself is interesting, especially if you visit the museum inside the building.
Regardless of all the controversy surrounding the Sagrada Familia it is a truly magnificent building and an absolute must-see when you visit Barcelona.