The objectives of this part of lSO/IEC Guide 43 are: a) to establish principles for the selection of proficiency testing schemes for use in laboratory accreditation programmes; and b) to assist in harmonizing the use of results of proficiency testing schemes by laboratory accreditation bodies. As results from proficiency testing schemes may be used in accreditation decisions, it is important that both the accreditation bodies and participating laboratories have confidence in the design and operation of the schemes. It is also important for participating laboratories and laboratory accreditation assessors to have a clear understanding of the accreditation bodies’ policies for participation in such schemes, the criteria they use for judging successful performance in proficiency testing schemes, and their policies and procedures for following up any unsatisfactory results from a proficiency test. It should be recognized, however, that laboratory accreditation bodies and their assessors may take into account the suitability of test data produced from other activities apart from proficiency testing schemes. This includes results of laboratories’ own internal quality control procedures with control samples, comparison with split-sample data from other laboratories, performance of audit tests with certified reference materials, etc. The use of data from these sources by laboratory accreditation bodies is not covered by this part of lSO/IEC Guide 43. However, the principles set out in this part of lSO/IEC Guide 43, regarding follow-up of unsatisfactory performance, could also apply to these activities.