Stellar magnetic fields are directly detected or inferred across the whole Hertzsprung-Russell Diagram. Attention in this chapter is concentrated on premain sequence and on late- and early-type main sequence stars, with some discussion also of red giants and white dwarfs, and with a brief reference to neutron stars. The current observational situation is reviewed, and some of the consequences for our ideas on star formation, on stellar structure and evolution and on stellar rotation are summarized.