In the most general terminology, a reference material is a substance for which one or more properties are established sufficiently well to calibrate a chemical analyzer or to validate a measurement process. A reference material is used in a decision process; hence, the requirement for reliability of the value of the property measured must be consistent with the risk associated with a wrong decision. Any use of a reference material depends on the ability to make valid inferences from the measurement results. This involves the tacit assumption or demonstrated evidence that the material is reliable and capable of challenging the measurement process. The terms certificate and certification merely refer to the documentation that supports the reference material. Guidelines for the content of certificates for reference materials have been prepared by the International Standards Organization. Stability studies should precede any certification measurements if there are any doubts on the matter.