This book presents a unified approach to the liquid state, focussing on the concepts and theoretical methods that are necessary for an understanding of the physics and chemistry of liquids. The authors do not attempt to cover the whole field in an encyclopedic manner. Instead, important ideas are presented in a concise and rigorous manner, and illustrated with examples from both simple molecular liquids and more complex soft condensed matter systems such as polymers, colloids or liquid crystals. After a general chapter introducing the liquid state, the book is in four parts devoted to thermodynamics, structure and fluctuations, phase transitions, interfaces and inhomogeneous fluids, and, finally, transport and dynamics. Each chapter introduces a new set of closely related concepts and theoretical methods, which are then illustrated by a number of specific applications covering a broad range of physical situations. Accessible to graduate students with a basic knowledge of statistical physics or physical chemistry, this advanced text will be useful to physicists, chemists or chemical engineers seeking a broad-scope introduction to the liquid state of matter, from thermodynamics and structure to transport properties and activated dynamics.