A comprehensive account of the phenomena that occur when simple gases interact with surfaces, this text takes a fundamental perspective. Physical adsorption involves atomic or molecular films bound to surfaces by less than 0.5 eV per particle. Physically absorbed thin films exhibit remarkably diverse properties and behave in a manner characteristic of two-dimensional matter. This exploration focuses on monolayer physics, emphasizing atomic rather than molecular adsorption. The phase diagrams of physically absorbed films are diverse and rich in structure because of the subtle and varied competition between the two interactions: the mutual interaction between adsorbed molecules, and the force binding each molecule to the surface. The authors explain the microscopic origin of these forces in terms of constituent electrons and nuclei. They then examine the structural and dynamical properties of these films in the context of atomic and solid-state physics, statistical mechanics, and computer simulations. This text will be of interest to research chemists, physicists, and engineers alike, as well as students in these fields. Key literature citations allow readers to trace important developments, and thought-provoking problems are addressed in detail.