Magnetic activity in cool stars

The solar magnetic structure is discussed as a framework for stellar magnetism. Ways of observing magnetic structure in cool stars are outlined with emphasis on observations of Ca 2, H, and K emission in main sequence stars. It is inferred that the chromospheric, transition-region, and coronal emissions are correlated manifestations of magnetic activity in stars with convective envelopes, and that the main level of activity depends on the rotation rate.