Understanding thermodynamics is critical to being able to progress in any area of chemistry or energy. Furthermore, sustainability is irreversibly tied to efciency, and efciency depends entirely upon the physical and chemical laws governing the energy conversion process, that is, thermodynamics. How much power can we derive from an engine? What is the maximum efciency for a photovoltaic device? Is ethanol a viable replacement for gasoline in terms of overall energy value? All of these are ultimately questions about thermodynamics. Equilibrium thermodynamics deals with processes that exchange heat or work (or both) in systems as they approach equilibrium, and heat and work are both manifestations of energy. The purpose of this chapter is to provide a basic overview of these concepts of thermodynamics in order to understand the efciency of energy conversion devices.