The thermalization of a large energy fluxWin a medium of density &rgr; leads to heat waves consisting of compression and expansion fronts, associated with substantial bulk motion. Particles can enter or leave the thermalization front at subsonic, sonic, or supersonic speed, allowing for a wide variety of wave modes. The wave mode depends on power input, thermodynamic response of the medium, and the boundary conditions. The intake Mach number mainly determines the degree of compression achieved across the front. Using macroscopic conservation equations the thermodynamic properties of the wave fronts are discussed in a pressure–density diagram and compared to other thermodynamic processes. The fluid dynamic properties are shown as a function of the absorbed power inputW/&rgr;. Finally, the hydrodynamic properties of the entire two front heat wave are presented for specified boundary conditions in the response planeh–W/&rgr;, showing the enthalpyhof the heated target material as a function of an external energy fluxW/&rgr;. Examples of the boundary conditions of free and retarded flow and heat transport due to inverse bremsstrahlung are discussed.