Despite more than one century of observational stellar spectroscopy, the resulting data is not available in an easily accessible format. The necessity of such basic information is greater than ever, because new wavelength regions (ultraviolet, infrared) are now accessible and modern receivers can only analyse short stretches of spectra, so that a careful pre-selection of strategic elements is mandatory. This book presents a summary of our knowledge of the behaviour of all chemical elements identified in stars, based on observations rather than on their interpretations. Whenever possible the behaviour is described quantitatively, with the help of equivalent widths in different types of stars, or different ionization stages, for both absorption and emission features. It will provide an authoritative reference book for the astrophysical community.