The thermodynamics of adsorption are discussed with an emphasis placed on phenomena vital to understanding gas adsorption on metal–organic frameworks (MOFs). This chapter starts by introducing adsorption phenomena. The second section describes isotherms with an emphasis on isotherm types according to IUPAC, the causes of hysteresis curves, an explanation of scanning curves, a discussion of the differences between absolute, excess, and net adsorption, and a description of how pore-size distributions are found from isotherm data. The third section describes isotherm models, highlighted by a derivation of the thermodynamic basis for theoretical isotherms, empirical isotherms, monolayer isotherms, multilayer isotherms, and virial expansion isotherms. The last two sections describe the enthalpy of adsorption and the prediction of multicomponent isotherms from single isotherm data.