This book presents a summary of our observational knowledge of the behavior of all chemical elements identified in stars. Whenever possible the behavior is described quantitatively, with the help of equivalent widths in different types of stars (e.g. dwarfs and supergiants), or different ionization stages, for both absorption and emission features. The authors also describe the behavior of isotopes, and there is a separate section on molecules in stars and circumstellar envelopes. This book will be a fundamental reference for observational astronomers and astrophysicists.