The theoretical blackbody has an emissivity of one. The requirements for a practical blackbody are: 1. emissivity near unity 2. uniform cavity temperature 3. known temperature 4. known limiting aperture area. Primary standard blackbodies operate at pure elemental freezing point temperatures with reverse conical cavities. Secondary standard blackbodies with reverse conical cavities have continuously adjustable temperature and are available for temperature ranges from 10 to 3300 K. Transfer of calibration between primary, secondary and working standards blackbodies is made using contact temperature sensors and radiometers. The conical cavity, V-grooved and multihexaprismatic surfaces are practical for working standards blackbodies. Conical cavity sources with cavity diameters from 0.040 to 4.0 inches are available. Extended area flat plate sources from 1 millimeter square to 2.3 meters square and larger are practical.