Experience shows that an assessment of the seismic capacity of an existing operating facility can be required for a number of reasons, for example identification of potential seismic vulnerabilities based on operating experience events or the periodic safety review programme. This publication covers the seismic safety evaluation programmes to be performed on existing nuclear installations in order to ensure that the required fundamental safety functions are available, with particular application to the safe shutdown of reactors. It includes lessons learned based on the IAEA Action Plan for Strengthening Nuclear Safety, following the Fukushima accident, and updated methodologies for seismic safety evaluation of nuclear installations.