The majority of massive stars are born in binary and higher-order multiple systems, which leads to two interesting consequences. First, most massive stars will exchange mass with a binary companion at some point in their life and, secondly, most supernovae are actually from such stars. Mass can be transferred stably from one star to the other, but it can also result in the entire merger of both stars. In all cases, the further evolution and final fates of stars are profoundly affected. In this talk, I will discuss how the formation of neutron stars and black holes differ in binary stars compared to single stars, and how this helps to understand the growing landscape of compact-object masses. In binaries that transfer mass stably, the mass donor loses its envelope. This not only determines the final supernova type (i.e. SN Ib/c vs SN II) but also affects the pre-supernova core structure. In fact, stripped stars may explode in supernovae and produce neutron stars rather than collapsing into black holes. We further predict that stripped stars lead to more energetic explosions and produce higher nickel yields. In stellar mergers, I will show how strong magnetic fields emerge, and how this may lead to the formation of magnetic massive stars and highly-magnetised neutron stars, so-called magnetars.