With the development and implementation of performance-based earthquake engineering, harmonization of performance levels between structural and non-structural elements becomes vital. Even if the structural elements of a building achieve a continuous or immediate occupancy performance level after a seismic event, failure of architectural, mechanical or electrical elements can lower the performance level of the entire building system. This reduction in performance caused by the vulnerability of non-structural elements has been observed during recent earthquakes in Italy and worldwide. Moreover, non-structural damage has limited the functionality of critical facilities, such as hospitals, following major seismic events.  The investment in non-structural elements and building contents is far greater than that of structural elements and framing. Therefore, it is not surprising that in many past earthquakes, losses from damage to non-structural elements have exceeded losses from structural damage. Furthermore, the failure of non-structural elements can become a safety hazard or can hamper the safe movement of occupants evacuating buildings, or of rescue workers entering buildings. In comparison to structural elements and systems, there is relatively limited information on the seismic design of non-structural elements. Basic research work in this area has been sparse, and the available codes and guidelines are usually, for the most parts, based on past experiences, engineering judgment and intuition, rather than on objective experimental and analytical results. Often, design engineers must start almost from square one after each earthquake event: to observe what went wrong and to try to prevent repetitions. This is a consequence of the empirical nature of current seismic regulations and guidelines for non-structural elements.