This chapter introduces the key elements of radiometer calibration. It presents an overview of the fundamental calibration concepts for radiometers designed to measure the wavelength distribution of solar radiation in a given wavelength interval or “band-pass.” The concept of calibration traceability addresses the needs for a process to achieve consistent measurements within stated uncertainties. A radiometer calibration provides the instrument-specific responsivity specifying the value for converting the radiometer output signal to irradiance and its estimated measurement uncertainty. Radiometer calibration is an important part of the overall operations and maintenance of a measurement station or network of stations to ensure data quality. Determining the optimum calibration interval can be based on initially scheduling annual or more frequent radiometer calibrations, depending on the degree and type of exposure of the radiometer. Clouds and atmospheric conditions significantly affect short and longwave radiation that, in turn, significantly affects the temperature of Earth and the atmosphere.