Massive stars play an essential role in the Universe. They are rare, yet the energy and momentum they inject into the interstellar medium (ISM) with their intense radiation fields and fast, isotropic radiatively driven winds dwarfs the contribution by their vastly more numerous low-mass cousins. This stellar feedback dominates the energy and momentum budget in star-forming regions and galaxies leading to gas ejection, which has important implications for star and galaxy formation. Massive stars form from the gravitational collapse of magnetized, dense, and turbulent molecular gas in massive pre-stellar cores, which are located in highly embedded environments. During their formation, feedback from their intense radiation fields, collimated protostellar outflows, and stellar winds can limit their growth by accretion. In this talk, I will show a series of radiation-magnetohydrodynamic (RMHD) simulations of the collapse of massive pre-stellar cores into massive stellar systems that include these feedback processes to demonstrate how stellar feedback can limit accretion onto massive stars and disrupt their natal environments. In addition, I will also discuss how stellar feedback from massive stars, which are born in clustered environments, affects the dynamics of HII regions that surround these clusters and can drive turbulence in young, star clusters and molecular clouds.