In order to gain a fundamental understanding of friction, one must understand how the energy associated with work performed to overcome the frictional force is converted into heat at the molecular level. One of the simplest possible geometries in which friction can occur, and thus be studied, is that of a fluid or crystalline layer adsorbed on the surface of an ideal, atomically flat crystal. This system is ideal for studies of “interfacial” friction—that is, friction attributable to atoms and molecules immediately adjacent to the plane along which sliding occurs. Moreover it is directly accessible to experiments with a quartz crystal microbalance (QCM) and to theoretical studies through analytic calculations or molecular-dynamics (MD) simulations. The geometry is vastly simpler than that of contact between macroscopic objects in which the friction necessarily reflects the collective behavior of a multitude of buried contacts. (See the article by T. Baumberger and C. Caroli in this issue.) It is also far simpler than a case in which shear occurs within the bulk of a material rather than being confined to an interface. (See the article by D.A. Rigney and J.E. Hammerberg in this issue.) Nonetheless even in this simple geometry, friction can be far from negligible. A remarkable range of shear stresses, from 10−2 to 1010 N/m2, have been measured by a variety of techniques for wear-free geometries involving contact between crystalline interfaces.