  Chemically peculiar (CP) stars are main-sequence A and B stars with
abnormally strong or weak lines for certain elements. They generally have
magnetic fields and all observables tend to vary with the same period.
Chemically peculiar stars provide a wealth of information; they are natural
atomic and magnetic laboratories. After a brief historical overview, we discuss
the general properties of the magnetic fields in CP stars, describe the oblique
rotator model, explain the dependence of the magnetic field strength on the
rotation, and concentrate at the end on HgMn stars.
