The role and importance of using of certified reference materials in the QA/QC system is presented. The requirements for the application of CRM in the laboratory, the criteria for their selection, and use of appropriate statistical tools in the use of the certified values are specified. Requirements for manufacturers of CRM are also described. The chapter also includes the following basic information: Definitions and classification parameters characterizing the RM, selection criteria, rules for the application of reference materials, certificate and the information contained for use of RM in the laboratory, and using of the certified value during the validation process.