The evaluation of diagnostic tests (DT) or procedures is not subject to uniform regulations. However, a thorough evaluation of DT which are intended for use in routine medical practice is advisable or even indispensible for economic, medical, and ethical reasons. This article addresses some important aspects of this evaluation: the facets of "evaluating a DT", common validity measures for qualitative and quantitative DT, the applicability and generalizability of estimates of diagnostic accuracy, objectives and design of studies for evaluating DT, as well as specific methodological problems of theses studies.