Entropy is the key concept of thermodynamics, clearly explained and carefully illustrated. This book presents an accurate definition of entropy in classical thermodynamics that does not "put the cart before the horse" and is suitable for both basic and advanced university courses in thermodynamics. Entropy is the most important and simultaneously the most difficult term in thermodynamics to understand. Many students are discontent with its classical definition since it is either based on "temperature" and "heat," which cannot be accurately defined without entropy, or includes concepts such as "molecular disorder," which do not fit into a macroscopic theory. Physicists Elliott Lieb and Jakob Yngvason have recently developed a new formulation of thermodynamics that is free of these problems. The Lieb-Yngvason formulation of classical thermodynamics is based on the concept.