Physical adsorption involves atomic or molecular films bound to surfaces by less than 0.5 eV per particle (10 kcal/mole). The subject has attracted wide interest because these films exhibit behavior characteristic of two-dimensional matter. The phase diagrams of these films are diverse and often rich in structure because of the competing energy and length scales of the two interactions involved in this problem: the mutual interaction between adsorbed molecules and the force binding each molecule to the surface. This book describes our current understanding of these phenomena from a fundamental perspective. After explaining the microscopic origin of these forces in terms of constituent electrons, the text describes how statistical mechanical theory and computer simulation employ these forces to calculate the structural and dynamical properties of the films. Throughout the book comparison is made with many relevant experiments. Unique in its field, this book will be of interest to students and both academic and industrial researchers in chemistry, engineering and physics.