Restoration or strengthening of the historical masonry buildings in seismic areas involves both problems of philology and safety. Since the ancient techniques should be regarded as historical documents, like the architectonic feature of the buildings, they are worth being preserved. At this purpose it is firstly necessary to recognize them in their proper peculiarity and in reference to the historical evolution of the constructive techniques. This approach is the base for the design of interventions philologically correct. On the other hand the historical buildings are often in use like the modern ones; it follows that they require to be as safe, in seismic areas, as the new houses. At last it should be recognized that usual theoretical procedures for the structural analysis don't apply to the masonry structures, and safety verification can't be attained in the standard way. In this paper three fundamental principles are proposed, related to the mechanical behaviour of ancient masonry structures under seismic actions, and the way of ascertaining their safety or designing their restorations.