Abstract
A simple description is given of the quantum-mechanical theory of multiple scattering. The separation of the scattered wave into "coherent" and "incoherent" parts is discussed in greater generality than has been done previously. Applications to transport theory are described. Specific calculations are made of the refractive index of a medium which is "polarized" by the scattered particle (Lorentz-Lorenz formula) and also of a medium which has correlated structure (critical opalescence). Other applications are given.