In this chapter, we reprise Kirkwood’s model for a non-polar dielectricDielectricnon-polar material. In the Kirkwood model, a molecule in the dielectric is no longer representedDielectricpolar by a cavity and its neighbors are no longer represented by a surrounding continuous dielectric. Rather, the individual molecules are treated all on the same basis, being represented as point polarizable electric dipoles in a vacuumVacuum. The point electric dipoles interact pairwise by way of the electric dipole–dipole energy formula derived in Chap.  1 . With the separation between two interacting dipoles fixed, the effect of the remaining dipoles is replaced by a Boltzmann weighted average over their various possible positions and orientations. This thermal average cannot be calculated exactly but is represented by an approximation that is best described mathematically as replacing the average of a productProductcross of two quantities by the product of the two averages. This approximation is nonetheless effective, and leads directly to the Clausius–Mossotti equation.