Stellar analogues of the solar cycle and activity

The past two decades have seen a rapid increase in our understanding of the phenomenology of stellar activity. This has happened principally as a result of new instrumentation which has allowed studies at X-ray, EUV, visible and radio wavelengths in ways hitherto impossible. From this work has emerged a tentative description of a connection between activity, rotation, convection, and stellar structure and evolution, linked through pervasive electrodynamic processes. Although most manifestations of stellar activity studied to date occur with far greater amplitudes than the apparently analogous phenomena on the Sun, there is clear support for the notion that the ‘two-way street’ between studies of solar and stellar activity- the so-called solar-stellar connection - represents a useful path to improving our understanding of activity in general. In this paper we review the phenomenology of stellar activity with a special emphasis on those aspects that seem to be relevant in understanding the origin of activity cycles. We point out that the labile character of theories of solar activity makes it hard to synthesise a common account of activity in the Sun and other stars. Nevertheless, we show how the stellar data suggest that some aspects of activity (such as the rotation-activity connection and the non-linear correlations between different activity indicators) might be incorporated from the start into models of solar activity.