This paper examines the effects of viscous heating in the flow of Newtonian and non-Newtonian fluids. Two system were analyzed theoretically. One uses a Couette Flow model between two infinite parallel plates. The other examines Couette flow between concentric cylinders (infinite in length). It is found that, for the thermal boundary condition of constant wall temperature, the generalized Prandtl number (Prg) is the parameter which indicates whether or not viscous heating effects are important.