This publication elaborates the technical basis for the use of seismic isolation systems in nuclear installations. There are many benefits of seismic isolation in design and construction of new facilities and in the retrofitting of existing nuclear installations. Observations of well-designed, installed and maintained isolated structures show a no damage behaviour that corresponds to expectations. This publication presents the current status of practice and use of seismic isolation systems for nuclear installations. Since it relates to a number of IAEA Safety Standards, it complements these on the technical aspects related to usage of seismically isolation for structures, systems and components in nuclear installations. It will be of interest to all those concerned with seismic analysis, seismic design, and seismic safety evaluation for such installations.