The transport properties of fluids are, arguably, more sensitive probes of the forces between molecular species than are the equilibrium thermodynamic properties. The same properties are of engineering significance because they determine the size of process plant while equilibrium properties often determine feasibility. However, the measurement of the transport properties with an accuracy, which has enabled advantage of this circumstance to be taken, has proved rather more difficult and the industrial need for properties alone has seldom justified the effort. Lessons that have been learned about the measurement of the transport properties of simple fluids such as the monatomic gases and organic liquids have now provided the experience upon which to base measurements on less traditional materials under harsher conditions. The present paper reviews some of these historical lessons and shows how they have been applied to just one example from among current activities. The example concerns the measurement of the thermal conductivity of molten metals in a manner which can be validated at moderate temperatures and subsequently applied to more aggressive conditions.