There is a general agreement that auxiliary and supporting systems play an important role in the safe operation of a nuclear power plant. There is, however, no clear definition of such systems. The purpose of this publication is to propose a stepwise definition of auxiliary systems and supporting systems for nuclear power plants with water cooled reactors, describe the general design concepts and design recommendations that are common to these systems, as well as to provide recommendations on specific design considerations for a selection of auxiliary and supporting systems.