Toward Understanding Massive Star Formation Hans Zinnecker and Harold W. Yorke Abstract: The processes that produce massive stars are not well understood. Observations are challenging due to large distances, high extinction, and short timescales. Theoretical models remain controversial. This review describes the collapse of a massive molecular core and discusses three competing concepts of massive star formation: monolithic collapse in isolated cores, competitive accretion in protocluster environments, and stellar collisions and mergers in dense systems. It also reviews observed outflows, multiplicity, clustering, and the upper initial mass function and mass limit. The conclusion is that high-mass star formation is not merely a scaled-up version of low-mass star formation but a distinct mechanism, primarily due to stellar mass and radiation pressure. Key Words: accretion, circumstellar disks, HII regions, massive stars, protostars, star formation The review discusses the formation of massive stars, their observable stages, initial conditions, endproducts, and the role of feedback and triggering. It covers the basic theory of massive star formation, including the compression, collapse, accretion, and disruption phases. The review also discusses competing concepts of massive star formation, binary and multiple systems, and the universality of the upper initial mass function. It concludes that high-mass star formation is not a scaled-up version of low-mass star formation but a distinct process, primarily due to the role of stellar mass and radiation pressure. The review also discusses the future of massive star formation research and the importance of understanding the formation of massive stars for astrophysics and cosmology.Toward Understanding Massive Star Formation Hans Zinnecker and Harold W. Yorke Abstract: The processes that produce massive stars are not well understood. Observations are challenging due to large distances, high extinction, and short timescales. Theoretical models remain controversial. This review describes the collapse of a massive molecular core and discusses three competing concepts of massive star formation: monolithic collapse in isolated cores, competitive accretion in protocluster environments, and stellar collisions and mergers in dense systems. It also reviews observed outflows, multiplicity, clustering, and the upper initial mass function and mass limit. The conclusion is that high-mass star formation is not merely a scaled-up version of low-mass star formation but a distinct mechanism, primarily due to stellar mass and radiation pressure. Key Words: accretion, circumstellar disks, HII regions, massive stars, protostars, star formation The review discusses the formation of massive stars, their observable stages, initial conditions, endproducts, and the role of feedback and triggering. It covers the basic theory of massive star formation, including the compression, collapse, accretion, and disruption phases. The review also discusses competing concepts of massive star formation, binary and multiple systems, and the universality of the upper initial mass function. It concludes that high-mass star formation is not a scaled-up version of low-mass star formation but a distinct process, primarily due to the role of stellar mass and radiation pressure. The review also discusses the future of massive star formation research and the importance of understanding the formation of massive stars for astrophysics and cosmology.