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This chapter reviews the key thermodynamics concepts and formulae needed for understanding the atmosphere–ocean system. Some relevant aspects of the first and second laws of thermodynamics are summarized, with applications to the atmosphere and ocean. The thermodynamics of the ocean are presented and discussed, including the sources and importance of density variations and vertical instability. A simple conceptual climate model is derived to illustrate the importance of ocean thermodynamics to climate sensitivity. The vertical profiles of temperature, pressure, and density in the atmosphere are then revisited. The thermodynamics of dry atmospheres are presented, including the concepts of buoyancy and stability and the nature of convection in dry atmospheres. Finally, the role of water in the atmosphere is described and the fundamental atmospheric thermodynamics are extended to moist atmospheres.