  The current generation of millimeter interferometers have revealed a
population of compact (r <~ 0.1 pc), massive (M ~ 100 Msun) gas cores that are
the likely progenitors of massive stars. I review models for the evolution of
these objects from the observed massive core phase through collapse and into
massive star formation, with particular attention to the least well-understood
aspects of the problem: fragmentation during collapse, interactions of newborn
stars with the gas outside their parent core, and the effects of radiation
pressure feedback. Through a combination of observation, analytic argument, and
numerical simulation, I develop a model for massive star formation by
gravitational collapse in which massive cores collapse to produce single stars
or (more commonly) small-multiple systems, and these stars do not gain
significant mass from outside their parent core by accretion of either gas or
other stars. Collapse is only very slightly inhibited by feedback from the
massive star, thanks to beaming of the radiation by a combination of
protostellar outflows and radiation-hydrodynamic instabilities. Based on these
findings, I argue that many of the observed properties of young star clusters
can be understood as direct translations of the properties of their gas phase
progenitors. Finally, I discuss unsolved problems in the theory of massive star
formation, and directions for future work on them.
