This chapter studies the scattering of a nucleon, electron, or pion by a complex nucleus and introduces two key concepts: the average field acting on a nucleon, and the saturation property of nuclear dynamics. While the black sphere provides a qualitative interpretation of the elastic scattering of nucleons from nuclei, it does not predict precisely the observed angular distributions. In addition, the radius required to give a best fit to the scattering from a given nucleus depends somewhat on the energy of the scattered nucleon. The success of the optical model reinforces our picture of a nucleus as a tiny drop of nearly incompressible matter. The hypothetical ideal of nuclear matter has been especially useful for theorists trying to find approximate solutions to the Schrödinger equation for nuclei. In a real nucleus, of course, the protons feel the electrostatic forces. The electrostatic potential can be measured independently by scattering electrons from nuclei.