The works of Costa Rican composer Julio Fonseca have been largely neglected in both performance and scholarship. Currently, just a handful of his more than two hundred works are performed. This dissertation restores the 1935 version of Fonseca's Mass in G (known as Misa en Sol) for chorus and orchestra. Included in the document is a reconstruction of the score from its manuscripts into an edited full score, individual parts for chorus and orchestra, a piano reduction, and a conductor’s guide and commentary. A discussion of the editing process is provided that outlines the mistakes, missing parts, and changes that were necessary to make the Mass in G performable. Two piano versions by the composer were compared with the new full score to correct any mistakes from the manuscripts. After a performance of the work using this new edition, a final performance edition has been created. Fonseca's Mass in G is a musical treasure from the Costa Rican repertoire. Prior to this project, this mass only existed in manuscript form since its composition in 1935, and there is not a known edition that provides easy access to performance. This new edition provides a reintroduction of the piece into the repertoire, not just in Costa Rica, but also internationally.