All reactions described in this book so far have been equilibrium processes, or at least so close to equilibrium that relations derived for equilibrium can be applied without any significant loss of precision. But interest in thermal properties are not restricted to situations of complete equilibrium. After all, complete equilibrium means that nothing happens, on the average. Expanding the topics covered into non-equilibrium states and processes adds a layer of complications relative to the equilibrium description, complications that often prevent a description which is simultaneously simple and accurate.