The methodology for inferring temperature from optical radiation exitent from an opaque surface is described. Beginning with the measurement equation, the output of the radiation thermometer is related to radiant flux reaching the detector. Characteristics of blackbody radiators are reviewed, and the effects of surface emissivity, reflected irradiances, and the atmosphere are presented. Applications to a reheat furnace and a direct-fired process heater illustrate the practical importance of these effects and point out the need for a firm understanding of the physics of thermal radiation to properly interpret radiation thermometer observations.