This paper reviews the literature on the influence of viscosity on separation in dense medium processes, for both baths and cyclones, in the context of the well-known fundamentals of particle motion in fluids. It is concluded that viscosity does play a role in determining both the separation density and the extent of separation inefficiency (Ep). because most DMS processes probably operate in the intermediate particle flow regime. A high viscosity increases particle misplacement in baths and cyclones. It increases or decreases the separation density of bath separators (depending on whether the bath is top or bottom fed. Respectively) and decreases the separation density in cyclones due to a reduction in the medium differential. The effects of viscosity are strongly size dependent. In practice, viscosity is not seen as a major problem in most coal preparation applications, as long as it is properly managed.