This document deals with official actions which may be undertaken by a State for the purpose of attributing a « legal » quality to measuring instruments. Every effort has been made to consider all possible methods involved in these official actions. The many possibilities offered should enable both States and the authors of International Recommendations, to select the solution or solutions to be adopted or recommended, according to the different types of instruments concerned, the methods used for their verification, use and maintenance, their reliability, administrative practices etc.