S. Longmore (Astrophysics Research Institute, Liverpool John Moores University, United Kingdom),
J. Alves (University of Vienna),
J. Bally (University of Boulder),
N. Bastian (Liverpool John Moores University, United Kingdom),
E. Bressert (CSIRO Astronomy and Space Science),
J. Dale (LMU, Germany),
D. Kruijssen (MPA, Germany),
J. Rathborne (CSIRO Astronomy and Space Science),
L. Testi (ESO, Germany),
A. Stolte (University of Bonn, Argelander Institut, Bonn, Germany)

We review the formation and early evolution of the most massive (and dense) young stellar clusters in our Galaxy, and the role this can play in our understanding of star and planet formation as a whole. We start by reviewing the global properties and environmental conditions of the known young massive clusters (YMCs) and their progenitor gas clouds. We then focus on the observed embedded dense core and (proto)stellar populations within these clusters, and discuss how such large stellar systems can potentially simultaneously distinguish between competing cluster formation models and provide the strongest constraints on the physical origin of the stellar initial mass function. After reviewing the current theoretical framework for the formation and evolution of YMCs, we summarise the debate on two key open questions: infant (im)mortality and the evidence for/against significant age spreads within YMCs. We then discuss which fraction of all stars and planets, including our own solar system, may have formed in extreme environmental conditions (proximity to high-mass stars, high (proto)stellar densities etc) similar to those found in these clusters, rather than the more benign conditions found in local, low-mass star formation regions. Finally, we review how the most massive and dense young clusters can be used as a bridge linking what we learn about star and planet formation in the nearest-by regions, to starburst systems in the Local Universe, globular cluster formation and star/planet formation at the earliest epochs of the universe.

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