An important new approach incorporated into several advanced LWR and HWR designs involves the use of passive safety systems. This approach is expected to provide a very high level of safety and improved economics through design simplification. Considering the weak driving forces of passive systems based on natural circulation, careful design and analysis methods must be employed to ensure that the systems perform their intended function. This publication describes the present state of knowledge of natural circulation in water cooled nuclear power plants and passive system reliability. It presents extensive information on phenomena, models, predictive tools and experiments that currently support design and analysis of natural circulation systems, and highlights areas where additional research is needed.