The study of magnetic structure and activity in cool stars is largely inspired by the Sun. In the Sun the magnetic field has been concentrated in discrete elements which form a sequence, from the large dark sunspots down to the bright small elements in the magnetic network. The field strengths range from 3 kilo-gauss in sunspot umbrae to about 1.5 kilogauss in the network elements (for reviews, see Harvey, 1977 and Zwaan, 1980). This sequence may be explained by a set of magnetostatic flux tube models which depend on the magnetic flux through the tube (for a summary and references, see Zwaan, 1978). A basic assumption in the magnetostatic modeling is that the energy flux within the flux tube is no more than 20% of the energy flux in the nonmagnetic convection zone.