This Safety Guide provides recommendations on the seismic safety evaluation of nuclear installations. It addresses all types of new and existing nuclear installations. This Safety Guide presents three assessment methodologies: the deterministic approach, generally known as seismic margin assessment (SMA), seismic probabilistic safety assessment (SPSA), and a combination of SMA and SPSA known as ‘probabilistic safety assessment (PSA) based SMA. This Safety Guide provides specific recommendations on applying a performance-based graded approach to the seismic safety evaluation of nuclear installations other than nuclear power plants. It also covers the relation between seismic safety margins and defense-in-depth (DiD) level 3 and level 4. For new nuclear installations, this Safety Guide provides recommendations to assess adequacy of seismic margin to avoid cliff edge effects considering DiD level 3 and level 4. This publication is intended for use by regulatory bodies, operating organizations, and designers of nuclear installations.