  Although the Sun is our closest star by many orders of magnitude and despite
having sunspot records stretching back to ancient China, our knowledge of the
Sun's magnetic field is far from complete. Indeed, even now, after decades of
study, the most obvious manifestations of magnetic fields in the Sun (e.g.
sunspots, flares and the corona) are scarcely understood at all. These failures
in spite of intense effort suggest that to improve our grasp of magnetic fields
in stars and of astrophysical dynamos in general, we must broaden our base of
examples beyond the Sun; we must study stars with a variety of ages, masses,
rotation rates, and other properties, so we can test models against as broad a
range of circumstances as possible. Over the next decade, an array of indirect
techniques will be supplemented by rapidly maturing new capabilities such as
gyrochronology, asteroseismology and precision photometry from space, which
will transform our understanding of the temporal variability of stars and
stellar systems. In this White Paper we will outline some of the key science
questions in this area along with the techniques that could be used to bring
new insights to these questions.
