Over the past decade, largely because of the pioneer work of F. J. Low in Tucson, it has become possible to make accurate and reliable astronomical measurements at infrared wavelengths as long as 20 µm. At such wavelengths we see solar system bodies not by reflected sunlight but by their own thermal emission. The larger asteroids, though subtending small angles, give signals at 10 µm comparable in intensity with those from the brightest stars. It is now possible to determine the absolute flux from such asteroids to an accuracy of about 10 percent. Because an asteroid might reasonably be expected to have no atmosphere and no internal source of heat, the thermal radiation it emits must just balance the solar radiation it absorbs, and the measured flux will depend only upon its size. Infrared measurements therefore provide an opportunity to determine the diameters of the brighter asteroids.