The primary purpose of this paper is to discuss the separation of travelling wave effects from the mass center to resistance center eccentricity effects, and to propose means by which the torsional spectrum (or a torsional seismic factor) could be incorporated in the seismic provisions of the National Building Code of Canada. The paper begins the treatment of the subject matter by presenting a detailed review of previous work on the effects of torsional ground motion, emphasizing the methods which have been proposed to develop torsional response spectra. The travelling wave assumption is critically reviewed and the effective phase velocities applicable for design purposes are discussed. This is followed by a simple analysis showing how the torsional spectra can be derived directly from the corresponding translational response spectra. Formulae separating the torsional input effects from the accidental eccentricity effects are presented and discussed. Finally, it is shown how the torsional spectrum can be incorporated within the framework of the response spectrum procedure of the 1985 edition of the National Building Code of Canada. Key words: seismic, earthquake, structure, dynamic, rotation, code, torsion, foundation, eccentricity, response spectrum, acceleration, velocity, design.