Since the mid-1980s there has been much discussion of the advantages of passive safety systems in advanced nuclear power plants (NPPs). It has been recognized that the application of passive safety systems can contribute to simplification and potentially improve economics of new NPP designs. However, this implies careful design and analysis methods to assure that these systems perform their intended functions. This publication draws on the studies of an IAEA coordinated research project on the topic and reports the findings of the latest research activities. It describes passive safety systems in a wide range of advanced water-cooled nuclear power plant designs, defines the thermal hydraulic phenomena associated with natural circulation phenomena and cross-links these phenomena with the passive safety systems.