InEntropy the previous Chap. 12 , the caloric equations of state for internal energy as well as for enthalpy have been introduced and derived mathematically. Furthermore, a new state variable, the specific entropy s, has been introduced. At that time, however, it has not yet been clear how entropy can be used to evaluate thermodynamic systems. In this chapter, a clarification is given as to why entropy is useful and why it is indispensable for describing energetic transformation. First, a comparison with the first law of thermodynamics is made so that entropy balancing becomes comprehensible: Unlike energy, entropy is not a conservation variable, since entropy can be generated in a system. Nevertheless, entropy can be balanced. A distinction is made between closed/open systems and thermodynamic cycles. In addition, a new state diagram, the T, s-diagram, is derived. Such a state diagram visualises the process variables specific heat q and specific dissipation \(\psi \) . Together with the p, v-diagram, which visualises the other process variable, namely the specific work, it is obviously an important diagram in thermodynamics. Finally, two new changes of state, namely isentropic and polytropic changes of state, are physically motivated and treated in this chapter.