Résumé
To improve our knowledge of O stars, their evolution, and physical processes associated with O stars and their descendants, it is crucial that we have reliable determinations of fundamental parameters, abundances, and mass-loss rates. Unresolved issues include discrepancies between evolutionary and spectroscopic masses, limitations in the accuracy with which we can determine effective temperatures, surface gravities, and abundances, and problems in mass-loss rate determinations. We address these issues and their causes.