In this chapter, we review how entropy S is introduced in thermodynamics and statistical mechanics. The definition of entropy is accompanied by the definition of temperature T. In thermodynamics, entropy and temperature are introduced through arguments based on heat flow in quasi-adiabatic processes (Carnot cycle): \(dS = \delta Q / T\) , where \(\delta Q\) is the amount of heat exchanged during the process. In statistical mechanics, entropy is derived from the principle of equal a priori probabilities, which leads to Boltzmann’s principle: \(S = k_\textrm{B}\ln W\) , where W is the number of microstates corresponding to a given energy.