To get a reasonably realistic picture of the structure and evolution of stars one needs to know accurately the abundances of chemical elements and their isotopes in the stellar atmosphere and in the galactic environment of the stars. The articles collected in this volume give a modern review of the abundance accuracies for main-sequence stars. After a general introduction in the first part the accuracies of atomic transition probabilities, ionization and excitation cross-sections, and line broad- ening data are discussed. In the second part the specific problems and results for different stellar types are presented, and, finally, an overview on the possibilities of abundance determinations for stars outside the Galaxy is given.