This book is suitable to be used as a text in introducing graduate courses or advanced undergraduate courses on the equilibrium statistical mechanics of bulk phases, interfaces, thin films and associations colloids. Emphasis is placed on exactly solvable models or physically motivated approximate theories that offer revealing insights into the molecular origins of the behaviour of materials in equilibrium. Also emphasized are theoretically motivated semiempirical models that can be used in quantitative predictions of phase and interfacial behavior. The book is unique in its unified approach to the theory of phases and their interfaces and on the density functional theory that unification is based on.