Critical facilities are man-made equipments, plants, constructions, and structures that, if affected by a strong earthquake, can produce serious impacts on people, environment, and economy. Therefore, for these facilities specific provisions in terms of seismic design are required and detailed seismic hazard evaluations have to be developed. In this paper, firstly the concept and meaning of ``critical facilities'' is argued. Then, a focus on the seismic hazard of nuclear power plants is presented since this type of critical facilities could be considered the facilities that more than others have contributed to define the most advanced knowledge in the field of seismic hazard assessment.