The ability to predict sliding friction in an adsorbed film system is key to a vast range of fundamental and applied issues in physics and nanotechnology. Sliding friction levels in molecularly thin layers can be measured experimentally by means of the quartz crystal microbalance (QCM) technique, which is readily comparable to theory, including both analytic calculations and molecular dynamics simulations. Progress in this area is summarized here, focusing on fundamental energy dissipation mechanisms and how nanoscale observations might be linked to the macroscopic world.