With the challenges our society faces concerning energy and the environment, it is particularly vital to develop a more rational use of our resources, both renewable and non-renewable. A sustainable use of the available resources is only possible if engineers can rely on coherent indicators, among which the exergy efficiency is bound to play a major role. The goal of this book is thus to facilitate the teaching and understanding of thermodynamics for students of engineering. A large part of the book handles the concept of irreversibility and the notion of entropy, with mathematical formulations that allow the concepts to be applied in an efficient and practical manner. In particular, a general theory of exergy is developed, a theory that simultaneously implements the First and Second Laws. Exergy accounting is the only way to accurately calculate the thermodynamic losses of a given process and to unambiguously define a thermodynamic efficiency expressing its level of perfection. Many applications are presented with the goal of illustrating the practical aspects of the theories described, including combustion chambers, boilers, turbines, compressors, motors, cycles, fuel cells, heat pumps and refrigeration units.