As the field of ionic liquids matures to more industrially implemented applications, robust models of their physico-chemical properties become necessary for process optimization. Viscosity is a particularly difficult property to model since there is no generally accepted theory for the viscosity of liquids. This paper aims to review the viscosity models developed or adapted to ionic liquids and their mixtures that are available in the literature. The scope of application and limitations of these models are discussed. In particular, the mixing rules' current formalism for ionic liquids is analysed in light of established knowledge from the inorganic molten salt community.