When different parts of a body are at different temperatures, heat flows from the hotter parts to the cooler. There are three distinct methods by which this transference of heat takes place: (1) conduction, in which heat passes through the substance of the body itself; (2) convection, in which heat is transferred by relative motion of portions of the heated body; and (3) radiation, in which thermal energy is transferred directly between distant portions of the body by electromagnetic radiation. We will not discuss convection in this work, and radiation transfer will be discussed in Chapter 9.

In general, a temperature gradient will be accompanied by a pressure gradient as can be seen in an equation of state, such as that of a perfect gas. In many cases hydrodynamic energy transport dominates over that associated with heat conduction. Thermal heat conduction transports energy comparatively slowly through a medium, while a small pressure difference causes disturbances to be propagated with the speed of sound, leaving a redistribution of density. Hence, the pressure equalizes more rapidly than the temperature. In high-energy-density matter one must consider both modes of energy transport, as well as radiation transport.