The Monastery of St. Catherine on Mount Sinai was built under the Byzantine emperor Justinian in the sixth century, on a site that had been venerated long before as the place where Moses encountered God in the form of the burning bush (Exodus 3:1-10). In the fourth century AD, a small shrine had stood there, built at the request of the Empress Helena, the mother of Costantine. For centuries, the high number of pilgrims, monks and hermits who had come to honor this holy place had endured the uninterrupted harassment of raiders and the harsh circumstances of the desert. This paper describes the conservation intervention of the mosaic of the Transfiguration, located in the apse of the basilica of the Monastery of St. Catherine's, as well as the methodological choices that have led to the conservation strategy that was adopted. As with all works of art, mosaics do not enjoy eternal life because of their very nature; to the contrary, they are fragile and need care and attention. Alarmed by clear signs of decay in the mosaic, the monastic community of St. Catherine's embarked on a conservation program and entrusted the work to the CCA, Centro di Conservazione Archeologica. The treatment, which began in November 2005, has been long, delicate and complex.