This chapter introduces the optical potential to explain the scattering of unbound nucleons by nuclei. It explores the two-way relationship between the motion of the nucleons in the nucleus, and the average forces which both determine, and are determined by, their motion. The quantitative interpretation of non-elastic reaction cross sections requires an extension of scattering theory to inelastic processes. A simple model has been very successful in explaining the single-particle aspects of atomic structure: the Hartree-Fock theory. The independent-particle model is quite successful at describing the motion of a nucleon in a nucleus, whether the nucleon is in a bound state or a continuum state. The most widely used method for taking account of the short-range correlations between nucleons in nuclei, the Brueckner theory, takes as its starting point the theory for scattering of free nucleons. A model which includes the rearrangement potential by way of the effective interactions’ density dependence is called Density-Dependent Hartree Fock.