The purpose of this book is to provide an outline of our knowledge about the behavior of the chemical elements in stars. As every observational spectroscopist knows, one is often confronted with essentially simple questions of the following kinds. What is the behavior of a given element in a given group of stars, for example, europium in metallic line stars or in S-type stars? Are the neutral lines of this element visible, are they strengthened or weakened with regard to those of normal dwarfs? Questions like these are often difficult to answer even for specialists and we have thus thought that it would be useful to collect the available information and to present it in such a way as to be useful for others.

We have reviewed the literature for both normal and non-normal stars, in the classical wavelength region (3800–4800 Å) as well as in the ultraviolet and the infrared (when available) for both absorption and emission lines. We have tried to stick as closely as possible to observations and to refrain from interpretation; this means for instance that we quote equivalent widths rather than abundances, whenever possible. The separation of observations from interpretation is especially useful in fields that are in a constant state of flux. This alludes for instance to interpretations of observed abundances in terms of the thermonuclear processes going on in the stars, or to interpretations involving physical processes like diffusion in stellar atmospheres or mechanisms for heating of the corona.