Orbital measurements of the Earth's long-wave emitted radiation, and the Sun's radiation reflected by the Earth have been made by scanning radiometers on three spacecraft platforms in both high-altitude Sun-synchronous polar (833 km) and low-altitude (600 km) equatorial orbits. The Earth Radiation Budget Satellite (ERBE) instruments have been designed to measure the radiance from the Earth with an absolute radiometric error of less than 1%. The scanner instruments consist of three broadband radiometer channels: short wave, 0.25 to 3.5 μm; long wave, 5.0 to 50 μm; and total, 0.25 to >50 μm, and use thermistor bolometers as sensing elements. These radiometers repetitively scan the Earth from limb to limb every 4 s producing contiguous pixels on the Earth's surface. Each scan contains views of cold space and of an internal calibration source, to obtain a radiometric reference and a calibration check, respectively. The design and operation of the ERBE scanner are described, and an overview of the ground calibration approach and inflight calibration stability is given along with an analysis of on-orbit anomalous behavior.