Few scientific notions have aroused the speculative imagination like the thermodynamic entropy. All organised systems - societies, living creatures - are destined without exception to decline and eventual death. This book clearly exposes the historical and conceptual development of thermodynamics. Born from a desire to understand and master steam powered machines - the symbol of our industrialised societies - it became the science of the human body. However, it was suddenly passed over in favour of the theory of atoms. It was thus demolished by statistical mechanics which ceded to the imperatives dictated by the atomical structure of the body. After an epic struggle, sometimes quite ferocious, thermodynamics and statistical mechanics have been reconciled by adopting the base of the second with the techniques of the first. This book reads like a novel about contemporary physics. Bernard Diu, a graduate of the Ecole Normale Supérieur, is a professor at the University of Paris VII.