Tracing the history of thought on the molecular origins of surface phenomena, this volume offers a critical and detailed examination and assessment of modern theories. The opening chapters survey the earliest efforts to recapture these phenomena by using crude mechanical models of liquids as well as subsequent quasi-thermodynamic methods. A discussion of statistical mechanics leads to the application of results in mean-field approximation to some tractable but artificial model systems. More realistic models are portrayed both by computer simulation and by approximation to some portrayed both by computer simulation and by approximations of the precise statistical equations. Emphasis throughout the text is consistently placed on the liquid-gas surface, with a focus on liquid-liquid surfaces in the final two chapters. Students, teachers, and professionals will find in this volume a comprehensive account of the field: theorists will encounter novel problems to which to apply the basic principles of thermodynamics, and industrial scientists will deem it an invaluable guide to understanding and predicting the properties of the interfacial region. Its extensive cross-referencing effectively assembles many diverse topics and theoretical approaches, making this book indispensable to all those engaged in research into interfaces in fluid-phase equilibria.