The uncertainty of measurement is the key indicator of the quality of any experimental result. Proper consideration of this uncertainty is imperative when testing a sample against legal/compositional limits. This task can be quite challenging when the entity measured in the investigated sample is so close to the limit that its uncertainty, however estimated, critically affects decision-making. This explains the many literature contributions discussing the problem. Even though some of the most authoritative organisations have issued specific guidelines aimed at assisting the staff involved in such measurements, several aspects of conformity testing are still debated in the literature. In this review, after a short outline of existing information, somewhat more detailed insight is given into the guidelines of ASME, ISO, and Eurachem/CITAC, because they are the most useful tools for operators of testing and calibration laboratories. Some aspects of Council Directive 96/23/EC are also discussed. Insight into the contents of the mentioned documents enables emphasis of analogies and discrepancies.