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The physical mechanism by which massive stars form is one of the outstanding problems in astrophysics, but one that has seen much recent progress thanks to an influx of new data. In the last few years, millimeter interferometers have revealed a population of compact (r <~ 0.1 pc), massive (M ~ 100 Msun) gas cores that could be the direct progenitors of massive stars. I discuss the evolution of these objects, focusing on the questions of whether they collapse to a few massive stars or many low mass ones, whether stars formed in this process undergo a phase of competitive accretion that determines the stellar IMF, and whether radiation pressure from newly formed stars is able to halt accretion. Based on a combination of analytic modeling and simulations, I argue that massive cores are indeed the precursors of massive stars, and that many of the observed properties of young star clusters can be understood as direct imprints of the properties of their gas phase progenitors.
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