Performance criteria should be a challenge for the laboratories to improve their quality. In countries with mandatory proficiency testing, the definition of performance criteria is a particular issue. If the definition of performance criteria is mandated from the regulatory bodies to medicoscientific institutions, scientific approaches (i.e., based on biological variation), the state-of-the-art approach (i.e., based on technical feasibility) as well as medical needs can be used to set up performance criteria such as the Richtlinie der Bundesärztekammer (RiliBÄK; Guideline of the German Medical Association on Quality Assurance in Medical Laboratory Examinations) in Germany. The experiences with RiliBÄK show that these performance criteria have to be revised on an ongoing basis.