AbstractThe effect of point defects on the thermal conductivity of an insulating crystal is discussed. The mechanical problem of the scattering of lattice waves by point defects can be solved exactly. The resulting cross sections occur in the Boltzmann equation for the phonon distribution, which can be treated by a variational procedure. In particular, a study is made of the effect on the thermal conductivity of isotopes, together with that of other scattering processes due to anharmonicities and the surface of the crystal.