In order to provide population inversion in QCLs, the scattering rates have to be engineered properly by designing the energy levels and wave functions. The scattering events interrupt the kinetics of electrons. Between the scattering events the acceleration is governed by the external fields. Due to scattering, electrons dissipate their momentum and energy and can change the subband. Scattering processes are determined in terms of transition probabilities Sαα′ between an initial state |α⟩ and a final state |α′⟩ per unit time. Various scattering mechanisms exist. In this chapter we will consider and discuss scattering due to lattice vibrations as well as interface roughness. More detailed derivations can be found in Appendix B.