In order to gain a fundamental understanding of friction, and the closely related phenomenon of lubrication, one must understand, at the molecular level, how the energy associated with the work to overcome friction is converted to heat.@footnote 1@ Such knowledge is key to understanding the rate at which an interface will heat, and in addition how chemical reactions and other physical processes triggered by heat will be affected by friction. One of the simplest possible geometries in which friction can occur, and thus be studied, is that of a fluid or crystalline monolayer adsorbed on an atomically flat surface. This geometry is experimentally accessible to experiments with a Quartz Crystal Microbalance (QCM), to numerical simulation techniques, and to analytic theory. Measurements of the tribological properties of "model system" and "real world" lubricants have been performed for rare gases, octane and TCP adsorbed on lead, iron and/or copper surfaces in open geometries (with QCM) and also in confined geometries (by bringing a STM tip into tunneling contact with the QCM electrode). Lead substrates are of particular interest on account the recent observation of superconductivity-dependent sliding friction. Iron and copper substrates are of interest for a variety of practical applications. Interaction potentials for adsorbed rare gases are known to a high degree of accuracy, allowing highly reliable comparisons of theory to experiment. TCP is meanwhile a "real-world" lubricant known for its demonstrated anti-wear properties for macroscopic systems. Although this lubricant has been the subject of much research for over 40 years, the atomic-scale details of its lubrication mechanisms are far from being satisfactorily understood. @FootnoteText@ @footnote 1@"Surface science and the atomic-scale origins of friction: what once was old is new again.", J. Krim, Surf. Sci. 500, 741 (2002)