An extension of classical hypersonic similitude is developed which takes into account the interaction effect of the displacement thickness of the boundary layer. A basic result of this viscous similitude is that the total drag including frictional drag obe}^s the classical similarity law for the pressure drag. Additional similarity conditions governing viscous effects must be imposed in this similitude. Underlying the similitude is a new hypersonic boundary-layer independence principle. According to this principle, the principal part of a hypersonic boundary layer with given pressure and wall temperature distributions and free-stream total enthalpy is independent of the (high) external Mach Number distribution outside the boundary layer. Various features of viscous hypersonic similitudes are discussed. I t is found, for example, that it applies to three-dimensional boundary-layer interaction effects on flat bodies, provided the concepts of strip theory may be applied, and provided the aspect ratio is an invariant.