This chapter concentrates on the thermodynamics of pressurized gas storage. The pressure and temperature dependency of the compressibility factor, the Joule-Thomson coefficient, and the real isentropic exponent of hydrogen and methane are examined and compared. These thermodynamic properties are especially important for pressurized gas storage. The chapter then introduces the thermodynamic processes that present borderline cases for compression and expansion. It also demonstrates different calculation methods by applying them to hydrogen and methane. The thermodynamic borderline cases for reversible compression are constituted by the isothermal process resulting in the minimum required work and the isentropic process resulting in the maximum required work. To demonstrate the applicability of the thermodynamic model two different examples were selected. In the first the refueling of a vehicle storage tank is examined and in the second the pressure and temperature change within a salt cavern is analyzed.