Book excerpt: The International Commission on Radiological Protection (ICRP) has developed and systematically updated the system of radiological protection, which now recommends optimisation of protection measures within or guided by appropriate restrictions, such as dose constraints or reference levels, in all circumstances. This applies to all exposure situations (planned, emergency and existing) and all categories of exposure (occupational, medical, and public). Optimisation of protection is intended to reduce exposures to levels that are as low as reasonably achievable, economic and societal considerations being taken into account, and to manage medical exposures commensurate with the medical purpose.