The Health and Safety Executive (HSE) published its revised Safety Assessment Principles for Nuclear Facilities (SAPs) in December 2006. The SAPs are primarily intended for use by HSE's inspectors when judging the adequacy of safety cases for nuclear facilities. The revised SAPs refer in part to HSE's expectations relating to the technical discipline of radiation protection. The purpose of this paper is to describe for the benefit of a wider audience HSE's reasoning behind the final published SAPs and to set out the purpose of each specific radiation protection (RP) principle. The paper also discusses principles in other sections of the SAPs which are relevant to radiation protection. The paper notes that the SAPs should be viewed as a reflection of good practice in relation to nuclear facilities in the context of interpreting relevant parts of primary legislation such as the Nuclear Installations Act 1965.