This chapter outlines the seismic behaviour of the Rotunda of Thessaloniki, one of the most significant domed monuments of the Late Roman and Early Byzantine periods. In the light of the aforementioned remarks, a new scheme of general and local interventions was proposed, estimating that it can minimise the seismic sensitivity of the monument. For the analysis of the monument, the SAP90 program was used on the assumption of elastic behaviour. The monument was modelled as a 3D structure using shell elements for the masonry walls and frames for the columns. The building is the actual National Library of Greece, and therefore besides being a monument, it is being used every day by hundreds of people who, after the earthquake, were at great risk as it could be concluded from the pathology. The pathology recorded prior to the 1997 intervention is to be considered the summation of the building lifespan.