General properties of step heat waves are explained using the "response plane" which has axes of final enthalpy and absorbed power. As examples of heat waves we discuss the time development of ionizing radiation fronts in interstellar H II clouds, compare the description of chemical reactions in the response plane and the pv diagram, determine the operation conditions of steady fusion reactions, explain the time history of decaying laser sparks, and survey possibilities of thermonuclear fusion with lasers. All such heat waves exhibit similar fluid behavior since these phenomena occur due to the absorption of some energy into the ambient medium.