So far the law of energy conservation, i.e. the first law of thermodynamics, has been discussed, that quantified the changeability of energy. However, this changeability is limited, so that thermal energy can not be converted completely into mechanical energy in a steady state process. The second law of thermodynamics can be applied to evaluate the constraints of energy conversion. In Chap. 15 a clockwise Carnot machine was introduced: A machine that operates between two thermal reservoirs in order to convert thermal energy into maximum mechanical work. Its efficiency in best case is given by the minimum and maximum temperature the machine is working in-between. This principle is essential to understand the thermodynamic idea of exergy as maximum working capability of any form of energy. The significance of the exergy is presented in this chapter.