The precise determination of chemical abundances in stars is the key to a number of important disciplines in modern astrophysics including stellar evolution and nucleosynthesis as well as the chemical and dynamical evolution of the Galaxy. The basic transport mechanisms in stellar atmospheres rule the observational aspects of stellar surfaces which are manifest in stellar spectra. They involve radiation hydrodynamics, convection, and diffusion, and they must therefore in turn depend to some degree on the chemical composition of the stellar plasma. The normal appearance of a stellar spectrum implies that neither segregation of different elements nor surface mixing of isotopes produced or destroyed during the star’s lifetime contributes to the observed composition of the stellar surface. Observational peculiarities on the other hand often tell us about unusual conditions met in the outer layers of such stars, and thus provide new insight in processes never met under laboratory conditions.