[en] Traditionally, criticality safety has involved all fissile-material operations outside of reactors within the nuclear fuel cycle and weapons complexes. Activities include fuel enrichment, material conversion, blending, fuel-element fabrication, fresh and spent fuel storage, packaging and transportation, fuel recovery and reprocessing, and, more recently, long-term disposition. An analogous set of activities are involved in fissile-material operations performed in the nuclear weapons programs. Whether reactor fuel or special nuclear material, long-term disposition can include material stabilization, storage, and/or waste management. At present, commercial reactor fuel is being reprocessed in France, Russia, and the United Kingdom, with strong interest in fuel reprocessing in Japan and other Asian countries. In the United States, fissile-material stabilization and fuel 'conditioning' involve many of the nuclear chemistry processes originally developed for reprocessing. The objective of criticality safety is to ensure that the fissile material is safely '... subcritical both normal and credible abnormal conditions'. These lecture notes reflect the diversity of activities encompassed in the neutron physics aspects of nuclear criticality safety. (author)