Chemically peculiar hot stars

The upper main sequence is home to a diverse family of chemically peculiar stars, including λ Boo, Am-Fm, Bp-Ap, HgMn, He-weak, and He-rich varieties. This paper presents an informal review of the physical properties of these objects, including their location in the H-R diagram, frequency of incidence, rotation, binarity, magnetic fields, and variability. Part of the discussion is devoted to describing the bizarre surface compositions encountered in chemically peculiar stars, with an emphasis on insights provided by a generation of ultraviolet observations obtained with theIUE satellite. The paper concludes with an overview of the radiative diffusion mechanism which has been proposed as an explanation of the chemically peculiar phenomenon.