This Standard defines the use of tracer (dilution) methods in the measurement of single-phase fluid (gas or liquid) flows in closed conduits. This method of measurement is applicable only to single-phase homogeneous fluid mixtures. This Standard was developed to fill the need for a generalized reference based on fundamental principles to measure fluid flow using tracer methods. ISO standards issued in 1977 addressed tracer methods for gas flows; these were withdrawn in 2001, leaving a void on this subject. An internet search on this subject will find a large number of documents, standards, references, consultants, and manufacturers. Most of the papers, standards, and products are for very specific applications and provide detailed guidance only for those needs. This Standard defines the terms and principles needed for intelligent consideration of tracer methods for any application.