This Safety Guide was prepared under the IAEA safety standards programme for nuclear power plants (NPPs). The basic requirements for the design of safety systems for NPPs are provided in Safety Standards Series No. NS-R-1: Safety of Nuclear Power Plants: Design. This Safety Guide describes how the revised basic requirements should be met for the design of emergency power supply systems for NPPs.This publication is a revision of Safety Series No. 50-SG-D7, Emergency Power Systems at Nuclear Power Plants. It takes account of developments in the design of emergency power supply systems in NPPs since 1991 and includes recommendations and guidance on non-electrical power sources. This Safety Guide was prepared through three technical meetings and extensive review by experts from 21 countries over a period of four years.