J. Tan (University of Florida, Astronomy, Gainesville, United States),
M. Beltran (INAF-Osservatorio Astrofisico di Arcetri, Italy),
P. Caselli (University of Leeds, School of Physics & Astronomy, United Kingdom),
F. Fontani (INAF-Osservatorio Astrofisico di Arcetri, Italy),
A. Fuente (Observatorio Astronomico Nacional, Spain),
M. Krumholz (UC Santa Cruz, Astronomy, United States),
C. McKee (UC Berkeley, Depts. of Physics & Astronomy, United States),
A. Stolte (University of Bonn, Dept. of Astronomy, Germany)

The enormous radiative and mechanical luminosities of massive stars impact a vast range of scales and processes, from the reionization of the universe, to the evolution of galaxies, to the regulation of the interstellar medium, to the formation of star clusters, and even to the formation of planets around stars in such clusters. Furthermore, the synthesis and dispersal of heavy elements by massive stars plays a key role in the chemical evolution of the cosmos. Achieving a rigorous theoretical understanding of massive star formation is thus an important goal of contemporary astrophysics. This effort can also be viewed as a major component of the development of a general theory of star formation that seeks to explain the birth of stars of all masses and from all the variety of star-forming environments. Two main classes of theories for massive star formation are under active study, "Core Accretion" and "Competitive Accretion". In Core Accretion, the initial conditions of star formation are self-gravitating, centrally concentrated cores that condense from the surrounding, fragmenting clump environment with a range of masses. They then undergo relatively ordered collapse via a central disk to form a single star or a small-N multiple. In this case, the pre-stellar core mass function has a similar form to the stellar initial mass function. In Competitive Accretion, the material that forms a massive star is drawn more chaotically from a wider region of the clump without passing through a phase of being in a massive, coherent core. In this case, massive star formation must proceed hand in hand with star cluster formation. If stellar densities become very high near the cluster center, then collisions between stars could also be involved in forming the most massive stars. We review recent theoretical and observational progress towards understanding massive star formation, considering a range of observed galactic star-forming environments, physical and chemical processes, comparisons with low and intermediate-mass stars, and connections to star cluster formation.

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