Constrained layer damping treatments, consisting of alternate layers of viscoelastic and elastic materials, are being widely used in industry for vibration and noise control. Many efforts have been made over the past two decades to apply diverse mathematical techniques to the analysis of the dynamic response of various structures with such treatments added, either on the surface, or incorporated into the fabrication of the structure. It remains true, however, that the now classical equations of Ross, Kerwin, and Ungar (RKU) are the simplest, most readily used, and often the most reliable approach toward making such predictions, even in spite of the fact that the equations apply only to beams or plates with pinned edges. While these equations are strictly limited to this type of boundary conditions other classical boundary conditions have commonly been accounted for simply by defining an appropriate modal semiwavelength in terms of the eigenvalue and the actual length. This seems to work quite well in most cases, but breaks down quite badly for the fundamental mode of a cantilever beam. This is particularly unfortunate, since cantilever beams are becoming very widely used for measuring the damping properties of viscoelastic materials. The aim of this paper will be to (i) discuss the derivation of equivalent modal wavelengths for nonpinned boundary conditions; (ii) describe the reduction of data from tests on cantilever and clamped‐clamped beams with multiple layer damping treatments added in order to determine equivalent free layer properties; (iii) use the equivalent free‐layer approach and the RKU approach to predict modal damping of some particular structures, making use of given complex modulus properties of the materials selected for the viscoelastic layers, and compare the two analyses with measured results; and (iv) discuss a computer program to optimize the geometry of constrained layer treatments for beams of various cross sections and for plates.