Thermal diffuse scattering is an important inelastic-scattering process in electron diffraction. Since the energy loss from phonon-scattered electrons is much smaller than 0.1 eV, which is rather small in comparison to either the energy resolution of the current available energy filter or the energy spread of the electron source, it is probably impossible to separate the intensity of elastically scattered electrons from that of the TDS electrons by using an energy filter. Therefore the contribution of TDS electrons has to be considered in quantitative data analysis in electron microscopy. This chapter reviews some theoretical approaches for describing the diffraction of TDS electrons based on the semiclassical model. We first use the two-beam approximation to illustrate the basic principle for dealing with TDS. Then the theory is extended to the many-beam case. The coherent effect in TDS is examined, and a coherent length is defined. Finally we discuss the contribution of Huang scattering in electron diffraction pattern. Several other dynamic theories of TDS are given in Chaps. 8 – 11 .