The apse mosaic of San Teodoro presents numerous complicated problems. It has been heavily altered over the centuries, and the extent of these alterations has seriously impeded scholarly study. Comparison of the known extant antiquarian drawings, together with important unpublished archival documents, permits the reconstruction of its conservation history. This is considerably more complex than had been thought previously. The identification of at least four extensive, and often clumsy, restoration campaigns produces a new and precise definition of the parts of the original mosaic that survive. These manifest a level of quality scarcely imaginable earlier, and allow new technical and stylistic comparisons to be made in order to verify the commonly accepted chronology. The new documentation yields important information about the apse mosaic itself, and throws light on the history of Roman restoration workshops, mosaic-workers, materials and setting techniques.