The method for the determination of the shear stress distribution in the walls of a thin-walled closed section beam is described as is the determination of the twisting and warping displacements. The nature of the warping displacements in the walls of a thin-walled rectangular section box is discussed and the condition for the zero warping of a beam section established. Expressions for the stress distributions, the rate of twist and the warping displacements in a thin-walled open section beam are also derived.