This IAEA Safety Guide, which is jointly sponsored by the World Meteorological Organization, provides recommendations and guidance on how to comply with the safety requirements on assessing hazards associated with meteorological and hydrological phenomena. It includes the state of practice in the international community for dealing with these external natural hazards, considering the lessons learned from recent catastrophic event, as well as from new findings on climate variability. Furthermore, this publication provides recommendations on how to determine the corresponding design basis for these natural hazards, and recommends measures for protection of the site of a nuclear installation against hazards of this type. This Safety Guide is intended for use by regulatory bodies, designers of nuclear installations and operating organizations responsible for the safety of installations and for the protection of people and the environment from harmful effects of ionizing radiation.