In our scheme of developing classical diffusion, we naturally began from the view of continuum diffusion in which the position and momentum of a tagged, diffusing particle are continuous variables-which is what the spirit of Brownian motion entails. That indeed was our plan in Chapters 1 through 7. However, we noted in Chapter 7 that in order to describe diffusive jump motion in solids we need discrete jump models in which momentum plays no role. These discrete models were constituted on the basis of stationary Markov processes that were also at the core of continuum diffusion and dealt with via Langevin equations or Fokker-Planck equations.