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.

Traditionally, radiative transfer has been the domain of astrophysicists and climatologists. In nuclear technology one has been dealing with the ana- gous equations of neutron transport. In recent years, applications of radiative transferincombustionmachinedesignandinmedicinebecamemoreandmore important. In all these disciplines one uses the radiative transfer equation to model the formation of the radiation ?eld and its propagation. For slabs and spheres e?ective algorithms for the solution of the transfer equation have been ava- able for quite some time. In addition, the analysis of the equation is quite well developed. Unfortunately, in many modern applications the approximation of a 1D geometry is no longer adequate and one has to consider the full 3D dependencies. This makes the modeling immensely more intricate. The main reasons for the di?culties result from the fact that not only the dimension of the geometric space has to be increased but one also has to employ two angle variables (instead of one) and very often one has to consider frequency coupling (due to motion or redistribution in spectral lines). In actual cal- lations this leads to extremely large matrices which, in addition, are usually badly conditioned and therefore require special care. Analytical solutions are not available except for very special cases. Although radiative transfer problems are interesting also from a ma- ematical point of view, mathematicians have largely neglected the transfer equation for a long time.