Irreversibility plays an important role in evaluating thermodynamic processes. In case friction occurs, a system can not be operated reversibly. This has been treated in Chap. : A wire pendulum for instance does not reach its initial, starting position after a number of oscillations. The amplitude decreases by time until the pendulum finally stops in its rest position. This is due to dissipation, i.e. friction at the mounting point as well as due to interactions between environment and pendulum. Consequently, kinetic energy is dissipated and transferred to the environment from where it can not be gained back into the system. Consequently, the state value entropy in such a case rises due to dissipation. It has been shown, that the rate of generation of entropy is a quantitive measure for the degree of irreversibility.