  Magnetic A stars represent about $5\%$ of the upper main sequence stars and
exhibit highly ordered, very stable and often very strong magnetic fields. They
frequently show both, brightness- and spectral line profile variations
synchronised to stellar rotation, which are believed to be produced by atomic
diffusion operating in the stars' atmospheres, that are stabilized by multi-kG
magnetic fields. In recent years, with the development and the application of
the Doppler- and magnetic Doppler imaging technique and the availability of
high precision spectroscopic and spectropolarimetric data, it has became
possible to map chemical abundances and magnetic field structures of Ap stars
simultaneously and in more and more detail, based on full Stokes vector
observations. Here I will review the state-of-the-art in understanding Ap star
spots and their relation to magnetic fields, the development of Doppler- and
magnetic Doppler imaging into one of the most powerful astrophysical remote
sensing methods and the physics of Ap stars atmospheres we can deduce from the
simultaneous mapping of magnetic field structure and chemical abundances.
