Massive stars dominate the appearance and the evolution of galaxies. Despite their prominent role in shaping on the dynamics and chemistry of interstellar medium, their birth is still poorly understood. In the Milky Way, most young massive stars are found in parsec-scale molecular clumps with a cluster of companion stars. Therefore, massive star birth is linked to the collapse and fragmentation of molecular clouds and clustered star formation. It appears that massive star formation is about fighting against the odds every step in the way. The physical conditions (temperature and density) in a molecular clump limit the Jeans mass to about 1 Msun. This creates the first barrier for massive star formation since dense cores much larger than 1 Msun tend to further fragment into lower mass cores. Once protostars reach 8-10 Msun, the radiation pressure may halt the infall and prohibit stars from further mass growth. As protostars reach more than 15 Msun, significant ionization may disrupt mass accretion as HII regions develop and expand.

In this talk, I will focus on the early stages of massive star formation: the formation of massive cores. I will review recent studies of massive infrared dark clouds that reveal fragments of super Jeans mass at the onset of cluster formation. I will present measurements of dust polarization in massive molecular clumps, which suggest that magnetic fields play an important role during the collapse of molecular clumps and the formation of dense cores. I will discuss the implication of these findings to current theoretical ideas of massive star and cluster formation.