The thermal conductivities of five samples of commercial glasses and glass ceramics were measured at liquid helium temperatures. It was found that the conductivity data could be accounted for by an intrinsic structure scattering of phonons as proposed by Klemens combined with an internal boundary scattering. The intrinsic structure scattering of these specimens was less than that calculated by Klemens using Berman's data for vitreous quartz. The mean free path calculated for scattering at internal boundaries was of the order of 10-2 to 10-3 cm. © 1962 The American Physical Society.