We review the physics underlying thermal infrared emission, focusing on the processes that generate polarization. We also discuss a number of features of the theory that are not widely discussed in the literature, including the Straubel theorem for absorbing media, the mixed Poynting vector, and the emission of nonisothermal media. Using a model for emission from nonisothermal media, we present a quantitative discussion of the conditions under which Kirchhoff’s law is valid. Using this model, we show how it is possible for many blackbody simulators and infrared camera shutters to produce emission very close to true blackbodies.