Swiss nuclear power plants (NPPs) are amongst the most earthquake-proof structures in Switzerland. They have been designed, built and upgraded so that they can withstand even strong earthquakes. Their safety is subject to constant checking using state-of-the-art analysis methods. To comply with an ENSI requirement, the nuclear power plant operators carried out a thorough investigation into the seismic hazard at the NPP sites from 2001 to 2004 within the scope of the PEGASOS (German acronym for «probabilistic seismic hazard analysis for Swiss nuclear power plant sites») project.