From the kinetic theory of gases, thermal conductivity of principal carrier i (p, e, f and ph) is k i = 1 3 n i c v , i u i λ λ i , where n i is the carrier density and the heat capacity is per carrier, u i is the carrier speed and λ i is the mean free path (distance traveled by carrier before an scattering event). Thus, the larger the carrier density, heat capacity and speed, and the less significant the scattering, the higher is the conductivity. For phonon λ p represents the interaction (scattering) kinetics of phonons and is related to the scattering relaxation time τ p or rate (= 1/ τ p) through λ p = u p τ p. Phonons interact with other phonons, and with electrons, boundaries, impurities, etc., and λ p combines these interaction mechanisms through the Matthiessen rule. At low temperatures, scattering by boundaries is dominant and with increase in temperature the interaction rate with impurities, electron and other phonons become important, and finally the phonon-phonon scattering dominants for T > 0.2 T D. The interaction rates are reviewed in and includes quantum perturbation theory and MD.