I will review our current understanding of magnetic activity in cool stars. Young, rapidly rotating stars display magnetic activity signatures that are several orders of magnitude larger than those seen on the Sun. I will discuss how we can use spectroscopic techniques to map magnetic fields at the surfaces of young cool stars using the example of the K0 dwarf, AB Doradus. I will show how the magnetic field patterns of this star differ from those observed on the Sun and discuss the implications of these differences. The surface magnetic field maps we obtain can also be used to predict coronal properties of these stars, such as the extent of the closed corona and the amount of rotational modulation expected in X-ray lightcurves.