In this chapter we define the concept of an entropy calibrator. By using singlesystem entropy functions and an auxiliary system calibrated with the main system, entropy functions are defined which apply to all simple systems. It is shown that two entropy functions exist which are unique, additive and extensive and have the right properties with respect to adiabatic accessibility. We show that a Lieb–Yngvason machine can be used to obtained entropy consistency between systems and finally discuss the problem of finding entropy calibration states.