The viscosity of bitumens/heavy oils may be related to the tendency of the asphaltenes to form aggregates. This paper concentrates on the effects of asphaltenes on bitumen/heavy oil viscosity. The macro and micro structural arrangements of heavy oils/bitumens are described and their effects on viscosity discussed. Experimental evidence suggesting viscosity to be not only a function of asphaltene concentration but, more importantly, to be a function of the nature of asphaltene aggregates is shown. A new equation quantifying the influence of asphaltenes on viscosity is presented and compared with experimental data.