The theoretical models for the viscosity of concentrated emulsions are reviewed. All models predict that the relative viscosity of an emulsion, at low capillary numbers, is a function of dispersed-phase volume fraction and viscosity ratio (ratio of dispersed-phase viscosity to continuous-phase viscosity). The predicted values of relative viscosity from different theoretical models are compared with a large amount of experimental data. The cell model of Yaron and Gal-Or [Rheol. Acta 11 (1972) 241] predicts the relative viscosities of emulsions reasonably well over a wide range of dispersed-phase volume fraction and viscosity ratio. The cell model of Choi and Schowalter [Phys. Fluids 18 (1975) 420] overpredicts the relative viscosities, especially at high values of dispersed-phase volume fraction. The Phan-Thien and Pham model [J. Non-Newtonian Fluid Mech. 72 (1997) 305] underpredicts the relative viscosities of emulsions at high values of dispersed-phase volume fraction. (C) 2001 Elsevier Science B.V. All rights reserved.