This publication provides recommendations and guidance for meeting Requirement 32 of IAEA Safety Standards Series No. SSR-2/1 (Rev. 1), Safety of Nuclear Power Plants: Design, for optimal operator performance involving systematic consideration of human factors, including the human-machine interface (HMI). The Safety Guide provides a structured approach and guidance on application of human factors engineering (HFE) in the design of the HMI, which is the basis for human physical and cognitive processes in nuclear power plants. It applies to application of HFE in the design, operation and maintenance of the HMI for new plants, as well as for modifications of the HMI of existing plants.