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. ENSI’s assessment of the PEGASOS project was mainly positive. Based on the project results, ENSI took measures to further enhance the earthquake safety of the Swiss NPPs. ENSI also highlighted potential refinements which could help to decrease the substantial spread of the hazard results.