It’s often said that the origins of quantum mechanics can be traced to the year 1900 with Planck’s theory of black-body radiation. But where did that problem come from, that so ably exposed the limitations of classical physics? The term black body as a perfect absorber of radiant energy was introduced in 1860 by Gustav Kirchhoff, who applied thermodynamics to electromagnetic radiation, or heat radiation as it was called then. It’s not obvious that thermodynamics pertains to radiation because thermodynamics is concerned with the state of equilibrium, and radiating bodies are not normally in equilibrium with their environment. Kirchhoff examined the singularly important problem of cavity radiation, in which matter and radiation are in equilibrium. Cavity radiation is electromagnetic energy contained within a hollow enclosure bounded by thick opaque walls maintained at a uniform temperature. To observe cavity radiation, a small hole must be made in the walls surrounding the cavity so that some of it can escape. 1 As we’ll see, that hole plays a significant role in the theory, and we’ll refer to it simply as the “hole.” Cavity radiation is closely related to black-body radiation; see Eq. (5.3). In this chapter we consider cavity radiation as a thermodynamic system. The purview of thermodynamics is not limited to material systems; it applies to systems in equilibrium.