This monograph is devoted to long-range surface forces sig­ nificant far beyond a single monolayer and felt over tens or even hundreds of molecular layers adjacent to an interface. The transi­ tion from the concept of short-range effects that reigned earlier to the concept of long-range forces simultaneously signified the transition from a two-dimensional world to a three-dimensional one, incomparably richer in physicochemical phenomena. This transition took many years and evolved through many steps. It began with the Gouy-Chapman theory of diffuse ionic atmospheres, which together with London's theory of molecular forces was used as a basis for the development (beginning in 1937) of the DLVO theory of stability of lyophobic colloids. Further elaboration of the theory involved the introduction of new types of force, and a generalization (in 1954) to the case of interaction between unlike particles (hetero­ coagulation). This theory is fundamental in such large-scale prac­ tical problems as flotation, water treatment, dyeing, soil science, microbiology, and interaction between biological cells. This book is the first comprehensive monograph devoted to sur­ face forces. This fact makes it easier to attract the reader's interest; yet, the reader's demands become all the more difficult to satisfy completely. Indeed, the research that we review and analyze here covers about 50 years of work. Much data has been amassed, so that the main problem was a careful selection and an­ alysis.