The increased interest in the use of supercritical fluids (SCFs) technology brings with it renewed requirements for the thermophysical properties of such fluids and fluid mixtures for the determination of the feasibility and subsequent design of chemical process plant. The equilibrium properties of fluids generally determine the feasibility of a given process, whereas the transport properties of the same fluids dictate the sizing of process plant at the design stage. This chapter is concerned with the state of knowledge regarding the transport properties of fluids under supercritical conditions. Although a large amount of experimental data of these properties has been compiled, the diversity of the fluids and of the conditions of interest means that all of the information likely to be required in practice can never be generated by experimental means alone.