The objectives of nuclear criticality safety are to prevent a self-sustained nuclear chain reaction. This Safety Guide provides guidance and recommendations on how to meet the relevant requirements for ensuring subcriticality when dealing with fissile material and for planning the response to criticality accidents. The recommendations address how to ensure subcriticality in systems involving fissile materials during normal operation and during credible abnormal conditions, from initial design through commissioning, operation and decommissioning. This publication also provides recommendations on identification of credible abnormal conditions; performance of criticality safety assessments; verification, benchmarking and validation of calculation methods; safety measures to ensure subcriticality; and management of criticality safety. The guidance and recommendations are applicable to both regulatory bodies and operating organizations.