The internal structure of the Sun, consisting mainly of ionised hydrogen and helium, is described. Dynamo action occurs in the ‘convection zone’ – the outer 30% (by radius) of the solar interior – bounded below by the ‘tachocline’, a layer of strong differential rotation. The velocity field in the Sun as observed on the surface and as inferred by helioseismology in the interior is described; sunspots and the 22-year solar cycle are also described, as also the toroidal magnetic field as inferred from sunspot polarities. The general poloidal magnetic field of the Sun is described, with reference also to the surface granulation pattern; the phase coupling between toroidal and poloidal ingredients is noted. The fields of magnetic stars are described in terms of the oblique-rotator model, in possible conjunction with the dynamo process. The dynamo process in fully convective M dwarf stars is also noted. The interaction between the Sun and its planetary system is briefly surveyed; diffusion of particles across the magnetosheath leading to auroral phenomena are noted. The magnetic fields of galaxies, including the Milky Way, are described. Finally, the situation in neutron stars is considered, for which Hall currents within the crust influence the magnetic field evolution.