Thermal emission has been detected from the four largest asteroids: Ceres, Pallas, Vesta, Hygiea, at 1mm to 20cm and from a few smaller asteroids at 2cm. Such measurements place constraints on the thermal and electrical properties of those objects' surface layers. The existing observations indicate that the asteroids studied so far are covered with fine-grained regoliths, which are expected to result from meteoroid bombardment (``sand blasting'') of the surfaces over 100's of millions of years. Differences in the microwave spectra of Ceres, Pallas, Vesta, and Hygiea suggest variations in the dielectric properties of those regoliths and/or variations in the regolith thicknesses. However, those few objects represent just a tiny fraction of the thousands of main-belt asteroids, most of which are beyond the detectability of the current VLA. (A 100-km-diameter object at 1 AU distance from the Earth represents a flux of microJy at 2cm, decreasing to microJy at 8.4 GHz). In addition, there is an important gap in the wavelength coverage from 1 mm to 1 cm. Over this range, the spectra show a distinct drop in brightness temperature. The proposed upgrades in continuum sensitivity and the expansion of wavelength coverage should make a large number of these objects detectable at several wavelengths. Of particular importance is the 40-50 GHz band, which samples the wavelength gap between 1 mm and 1 cm.