  Massive stars have a profound influence on the Universe, but their formation
remains poorly understood. We review the current status of observational and
theoretical research in this field, describing the various stages of an
evolutionary sequence that begins with cold, massive gas cores and ends with
the dispersal and ionization of gas by the newly-formed star. The physical
processes in massive star formation are described and related to their
observational manifestations. Feedback processes and the relation of massive
stars to star cluster formation are also discussed. We identify key
observational and theoretical questions that future studies should address.
