The equations of state appropriate to the interiors of most stars are simple in one major respect: they may be derived using the assumption that the radiation, gas, fluid, or even solid, is in a state of local thermodynamic equilibrium or LTE. By this we mean that at a particular position in the star complete thermodynamic equilibrium is as very nearly true as we could wish. It is only near the stellar surface or in highly dynamic events, such as in supernovae, where this assumption may no longer be valid.