Solar and stellar coronae.

The existence of hot coronae throughout the HR diagram has been proved by the observations of the Einstein observatory since the late 1970s. This finding has stimulated a new interest in the study of stellar activity. The general picture relating stellar activity in late type stars to the internal generation, photospheric emergence, and chromospheric and coronal dissipation of magnetic fields, was confirmed by the X-ray observations; in addition, the large range in X-ray luminosity for stars of the same spectral class and luminosity type, related to their range in age, has shown that X-ray observations add new leverage to the study of stellar physics. In this paper, after a brief overview of stellar coronae, the author discusses how observations can be used to get insight on their structure.