The theory of simple and complex fluids has made considerable progress due to the emergence of new concepts and theoretical tools, the availability of a large body of new experimental data on increasingly complex systems, and far-reaching developments in numerical simulation. Two clear trends emerge from the present book, first the diversity of new and unexpected theoretical results relating to classical models of liquids; and secondly, the parallel emergence of new concepts, models and methods for the investigation of complex fluids and phenomena. The book lays stress on a tutorial presentation of the main topics, including liquid structure, metastability and phase transitions, confined fluids and interfaces, complex fluids and quantum fluids.