While there are a number of uses of interlaboratory comparisons, and variations in their design and im- plementation, it is still possible to specify the essential principles that need to be considered when organizing such comparisons. This part of lSO/IEC Guide 43 de- fines those principles and describes the factors which should be taken into account in the organization and conduct of proficiency testing schemes. Part 2 of lSO/IEC Guide 43 describes how laboratory accreditation bodies, who assess technical com- petence of laboratories, should select and use proficiency testing schemes. This part of lSO/IEC Guide 43 is intended for use by proficiency testing operators and users such as participant laboratories, accreditation bodies, regulatory authorities and clients of laboratory services who have a need to assess the technical competence of laboratories. It is particularly useful for laboratories in self-evaluation, but recognizes that proficiency testing is only one mechanism which can contribute to the establishment of mutual confidence between users of different testing laboratories.