Existing buildings, especially those constructed before modern seismic design provisions were adopted, are among the structures most vulnerable to collapse in strong earthquakes. They often lack adequate ductility, proper structural detailing and regular lateral load paths. Field surveys show that most human losses result from the collapse of buildings designed to outdated codes, or designed to modern codes but built with insufficient quality, rather than from the intensity of shaking alone. Several damage patterns recur in documented earthquakes. The most prominent is the soft storey, where inelastic deformation concentrates in an open ground floor used for parking or shops and the whole building collapses. Another is the short column, whose free length is restrained by infill walls and which therefore fails suddenly in brittle shear. Others are pounding between adjacent buildings with insufficient separation, and torsion caused by an offset between the centre of rigidity and the centre of mass. Unreinforced masonry (URM) buildings behave differently: because of weak connections between walls and floors, their walls collapse out of plane before their in-plane capacity is exhausted. The 1999 Kocaeli, 2011 Christchurch and 2023 Kahramanmaraş earthquakes confirm that the common factor is the gap between actual building behaviour and the behaviour assumed in design. Site effects also matter, including soil type, liquefaction and proximity to the fault, so peak ground acceleration (PGA) alone is not sufficient for assessment. A staged assessment is recommended. It begins with rapid visual screening in accordance with FEMA P-154, proceeds to linear analysis, and ends with nonlinear static (pushover) analysis for high-risk buildings. Eurocode 8 Part 3 (EN 1998-3) links the amount of information available about a structure to a confidence factor according to the knowledge level, from 1.35 for limited knowledge to 1.00 for full knowledge. It also requires brittle and ductile mechanisms to be verified separately. Retrofit strategies fall into three groups. The first increases strength and stiffness, for example by adding shear walls. The second increases ductility by jacketing columns with concrete, steel or fibre-reinforced polymer (FRP) wraps. The third reduces seismic demand through base isolation or supplemental dampers. The most suitable technique is chosen by considering cost, the impact on continued use of the building, and compatibility with the existing elements. In conclusion, the safety of existing structures depends on the accuracy of field surveys and in-situ testing, and on consistency between the target performance level, the analysis method and the retrofit technique. The structure should be re-analysed after intervention to ensure that weakness has not been transferred to another element. Presented by Assistant Lecturer Ahmed Saeed Hakim Department of Civil Engineering Source: ASCE/SEI 41-17, Seismic Evaluation and Retrofit of Existing Buildings, 2017; CEN, EN 1998-3, 2005.