JMMM 150 (1995) 13-24 Overview of interlayer exchange theory J.C. Slonczewski Basic mechanisms of interlayer coupling betwwen two ferromagnets separated by non-magnetic spacer are surveyed. Simple generic theoretical models yielding closed formulae are treated. First, intrinsic exchange between itinerant ferromagnets is trated in the free-electron band approximation, including corrections to the elementary sinusoidal dependences on metallic-spacer thickness and on angle theta included between the magnetic moments. Properties of metallic and insulating spacers are compared. Then three special mechanism of non-cos theta coupling are described: (1) fluctuations of spacer thickness, (2) loose spins, and (3) a novel phenomenological coupling through a non-normal spacer, possibly acting in spacers composed of chromium or manganese.