For steady state flow of fluid in a closed conduit, the only conserved parameter is the mass rate of flow, qm. If the mass density is known, the volume rate of flow qv can be deduced. The accuracy of flow rate measurement with the tracer methods is a function of how well the injected tracer material mixes with the flowing fluid. It is also a function of the accuracy and precision of the sensing devices and the (tracer methods) techniques used. Two tracer methods are used: a) The dilution method is based on a constant rate of tracer injection and the concentration of tracer found in the downstream conduit is a measure of the relative flow rates. b) The transit time method determines the flow rate by measuring the time it takes the tracer material to travel between two detector points or between the injection point and a detector point in the conduit. The advantages and disadvantages of these two methods are reviewed in Para 4. A wide variety of tracer materials may be used - radioactive or non-radioactive, mineral or organic materials, etc. The choice of tracer depends on the purpose of the measurement and environmental concerns(Para 5). The uncertainty of the measurements depends completely on the accuracy of the methods used (Para 7). Some typical tracer fluids are listed in Annex A.