In a simple and accessible form, this book presents a unified approach to the physics of the liquid state, both in and out of equilibrium. It discerns, behind the seemingly anarchistic proliferation of phenomena observable in the liquid state, the sequence of causes and effects and, where appropriate, the underlying rules that preside over the general principles. The book begins by introducing the fundamental concepts of statistical mechanics, such as classical and quantum mechanics, probability theory, and the kinetic theory of gases, before moving on to discuss theoretical methods in order to contextualise the study of liquids. The last final section is devoted to ordering in complex fluids. It includes detailed technical notes and explicit calculations, and will appeal to graduate students in physics and chemistry. It will also be of interest the reader interested in statistical mechanics and their application to the physics of dense matter. This book will certainly become an indispensable reference for students and researchers who wish to become familiar with a multifaceted process looking towards new horizons. Jean-Louis Bretonnet began his career as a Research Engineer at the Institut de la Recherche Sidérurgique Française in the field of metallurgy. After obtaining a PhD from the University of Metz, France, in 1978, he was appointed Professor of Physics at the same institution in 1985. His research interests focus on the physics of the liquid state, including liquid metals, simple liquids, and complex liquids.