The atmospheric activity of the sun and of stars of the solar-type is analyzed using observations from the HK-project at the Mount Wilson Observatory, California, and the Carnegie Planet Search Program at the Keck and Lick Observatories, as well as from the Magellan Planet Search Program at the Las Campanas Observatory. It is shown that a cyclical activity of the stars that is analogous to the 11-year solar activity cycle occurs in stars of spectral classes F, G, and K and is more pronounced in stars of class K. A comparative analysis of the solar-type stars with different levels of chromospheric and coronal activity confirms that the sun is one of the stars with a comparatively low level of atmospheric activity and that these stars have a minimal level of coronal emission and minimal variations in the fluxes of photospheric radiation.