The harmonisation of proficiency testing (PT) schemes has been under debate for a long time. There are obvious reasons why harmonisation of the practices in PT would be beneficial. In many areas, there is still a belief that further harmonisation of practices in PT would improve the comparability of measurement data. In particular when two laboratories are to be compared that have not participated in a single PT, problems arise which allegedly can be overcome by further harmonisation of PT schemes. In practice, however, parties involved in PT are not always embracing the idea of harmonisation. With the results of two European projects in mind, a discussion is given on harmonisation aspects, and some considerations are given that may help to decide in practice whether harmonisation is likely to solve particular problems. The first project, the European Proficiency Testing Network (EPTN), is concerned with further harmonisation. The second European project (COEPT) aims at providing a basis to assess equivalence across proficiency tests, and explores the conditions under which such an assessment is feasible.
