Abstract
Recent quiet sun observations in microwave regions are reviewed. In the review, the mechanism of radio emission
(thermal free-free) from the quiet sun and atmosphere models of the quiet sun are summarized. The models are
compared with the observations of the frequency spectrum of the disk center brightness temperature and of the
center-to-limb variations. Then the performances and limitations of the Nobeyama Radioheliograph (NoRH) for
studies of the quiet sun are summarized. By stressing the results from the NoRH, several observational features of
the quiet sun are summarized. Coronal holes and the polar cap brightening are particularly interesting subjects. It is
shown that the NoRH is a powerful tool for quiet sun studies.