The International Commission on Radiological Protection (ICRP) recommends a system of radiological protection that is in use worldwide. This paper describes the elements of that system, identifying the central assumptions and principles. A key assumption for protection at low doses is that there is a simple proportionate relationship between increments in dose and increments in risk. The system focuses on changes to the level of peoples' exposure, recommending that any change is justified (i.e., the benefit is clear) and protection is then optimised (i.e., improvement in dose reduction is promoted when possible and reasonable). In doing so, the system considers the amenability of the source of exposure to control and the acceptability of the exposure to individuals or society.