Stellar magnetic activity: the solar-stellar connection

The atmospheres of late-type stars are site of strong magnetic fields responsible for their non-radiative heating. Magnetic activity phenomena are often analogous to those observed on the Sun, but there are also phenomena which strongly deviate from the solar ones. Exploiting the solar-stellar connection is instrumental to a better understanding of the whole phenomenology of stellar activity. In this paper some of the results from the investigation on active cool stars are given: from the discovery of stellar activity cycles, to the characteristics of the active regions in the outer atmospheres. I also show how important is to understand magnetic activity phenomena when extrasolar planets are searched around solar-type stars.