As a result of all this activity in many different analytical sectors, together with the considerable amount of research that has been conducted, the analytical community has built up a large body of new experience with proficiency testing. It is pleasing to note that no substantive modifications of the fundamental ideas and principles of the 1993 Harmonized Protocol [1] are required to accommodate this new experience. However, the additional experience shows a need, and provides a basis, for refinement of our approach to many aspects of proficiency testing and for more specific and definite recommendation in some areas. Further, the original Harmonized Protocol was largely concerned with the organization of proficiency schemes, and is therefore addressed mainly to providers of schemes. increased importance of proficiency testing scheme data however, generated a need for additional guidance on the interpretation of results o schemes by both scheme ants and "end-users participa of analytical data (such as laboratory customers, regulators, and other stakeholders in laboratory quality). All these factors call for an update of the 1993 Harmonized Protocol.