The importance of medium properties in dense medium separation is now a widely accepted fact. It has been proven in the laboratory on a number of occasions that medium rheology is critical for efficient separation. However, transfer of the theory to the plant environment is not a simple exercise. The lack of instruments capable of measuring the rheology of industrial suspensions on a continuous basis has been one reason for the time it has taken for the importance of rheology to be recognised. This paper reviews some of the literature on viscosity measurement of suspensions in mineral processing and gives examples which demonstrate the effect of changes in viscosity on some mineral separations. The author argues that variations in local conditions preclude a universal model of medium rheology — particularly for contaminated medium - and that the alternative is for viscosity to be monitored directly on the plant.