This book grew out of the senior level lecture course I teach at Delft University and which I have taught in recent years at Eindhoven University and the University of Utrecht. Numerous discussions with students and colleagues led me to the conclusion that in spite of the existence of excellent books on the statistical theory of fluids, there is a gap between the fundamental theory and application of its concepts and techniques to practical problems. This book is an attempt to at least partially fill it. It is not intended to be a thorough and comprehensive review of liquid state theory, which would inevitably require invoking a large number of re­ sults without actual derivation. Rather I prefer to focus on the main physical ideas and mathematical methods of fluid theory, starting with the basic principles of statistical mechanics, and present a detailed derivation of results accompanied by an explanation of their physical meaning. The same approach applies to several specialized topics of the liquid state, most of which are recent developments and belong to the areas of my own activities and thus reflect my personal taste. Wherever possible, theoretical predictions are compared with available experimental and simulation data.