This study traces the history of ideas on the molecular origins of surface phenomena, critically expounds modern theories and assesses their present state. Early chapters survey the first attempts to describe these phenomena, in terms of crude mechanical models of liquids, and consider the quasi-thermodynamical methods that replaced them. A discussion of statistical mechanics is followed by the application of the statistical results in mean-field approximation to some tractable but artificial model systems. More realistic models are then treated both by computer simulation and by approximating the exact statistical equations. The emphasis throughout the text is on the liquid-gas surface, with attention given to liquid-liquid surfaces in the last two chapters.