Previous research efforts have shown that the rutting parameter used in the performance grade asphalt binder specifications, |G*|/sin ?, does not reasonably predict the rutting potential of asphalt mixtures, especially when modified binders are used. A number of other parameters, such as the zero shear rate viscosity and the permanent strain accumulated under repeated creep and recovery, were investigated; however, no consensus was achieved. This paper investigates the use of zero shear rate viscosity and of repeated creep permanent strain as potential specification parameters and discusses the importance of temperature susceptibility and of strain tolerance to the rut resistance of asphalt binders.