Stars are the building blocks of the Universe and have transformed the pristine Universe into the one we live in today. Massive stars in particular play a central role because they are cosmic powerhouses that give rise to copious amounts of ionising radiation, chemical elements and kinetic energy. They die as the most powerful explosions in the Universe and – if they lived in binary systems – their compact remnants are at the heart of gravitational wave astronomy. To understand the role of massive stars in the cosmos, a comprehensive
 picture of their lives and final fates is essential, but is as yet 
seriously incomplete. I will highlight some of the most pressing challenges in massive star research and then focus on two of them. First, I will discuss the stellar initial mass function of stars in the mass range 15-200 solar masses as observed in the local 30 Doradus starburst and implications for the maximum birth mass of stars and stellar feedback. Second, I will explain how stellar mergers may explain some of the enigmatic OBA stars that have strong, large-scale magnetic fields.