The occurrence of galling when unlubricated surfaces are slid together is shown through an energy balance to he determined by the parameter WKcot θ/p, where WKis the work of adhesion of the system, θ the average slope of the surface, and p the flow pressure of the softer metal. For lubricated surfaces, it is shown that only lubricants made up of very small molecules can be energetically stable on hard metal surfaces, and hence successful lubricants must form solid surface films. At the melting point of the lubricant film a transition takes place and the lubricant loses much of its effectiveness, while a second transition occurs at a higher temperature and leads to galling. This second transition is caused by desorption of the lubricant accompanied by a rise in WKbut varies widely for different metals, being influenced by their flow pressure.Contributed by the ASLE Technical Committee on Lubrication Fundamentals and presented at the Annual Meeting of the American Society of Lubrication Engineers held in Detroit, Michigan, April 1957.