The final analytical quality of a result is influenced by external and internal factors. The quality of external elements is influenced by the individual laboratory through its choice of method and supply; such elements are, e.g. reference materials, calibrators, control materials, reagents, reagent kits, instruments and disposables. The internal elements can be described through rules for good laboratory practice (GLP), and include all aspects of internal quality assurance. In this paper, GLP is used to describe the proper milieau in which the optimally selected method is working. The control system for acceptance or rejection of any particular batch of analysis, as well as the long-term control, are also described separately.