Phonon (quantized lattice vibration wave) is a central thermal energy carrier contributing to heat capacity (sensible heat storage) and conductive heat transfer in condensed phase, and plays a very important role in thermal energy conversion. Its transport properties are represented by the phonon conductivity tensor K p (W/m-K, from the Fourier law q k,p = - K p ⋅∇ T) for bulk materials, and the phonon boundary resistance AR p,b [K/(W/m)] for solid interfaces, where A is the interface area. The phonon specific heat capacity c v,p (J/kg-K) includes the quantum effect. The thermal energy conversion rate involving phonon is included in s ˙ ˙ i - j . Heat transfer physics describes and predicts, c v,p, K p, R p,b (or conductance G p,b) and s ˙ ˙ i - j , based on atomic-level properties.