Research of the Pontificio Istituto di Archeologia Cristiana on the Lateran Baptistery during the last two decades has resolved some of the many questions left open by the excavations inside the Baptistery in the 1920s and around it in the 1960s. This research has been coordinated by the author and Federico Guidobaldi and has involved the PIAC, the Swedish Institute of Classical Studies in Rome, the Vatican Museums and the Swedish National Heritage board. As a result it has been possible to determine the octagonal plan of the first (Constantinian) phase of the Baptistery, identify the foundations of the Oratory of Santa Croce, ascertain the height to which walls of the Constantinian phase are preserved, and deduce that the reconstructions attributed to the fifth-century Popes Sixtus III and Hilarus must be part of the same project.Laser scanned models and 3D documentation has been created as an instrument for research and for reconstructions. There remain, however, important, unresolved questions and these are also explored int his chapter.How was the building covered?Did it have an inner colonnade?What was the place of the first phase of this structure in the development of Late Antique architecture?