  Massive stars are born predominantly as members of binary (or higher
multiplicity) systems, and the presence of a companion can significantly alter
their life and final fate. Therefore, any observed sample of massive stars or
associated transients is likely to be significantly influenced by the effects
of binarity. Here, we focus on the relationship between massive binary
evolution and core-collapse supernova events. In the vast majority of the
cases, the first core-collapse event happening in a binary system unbinds the
two stars. Studying the population of companion stars, either at the supernova
site, or as "widowed" stars long after the explosion, can be used to constrain
the previous orbital evolution of the binary progenitor, and explosion physics
of their former companion. Specifically, the population of "widowed" stars
might provide statistical constraints on the typical amplitude of black hole
natal kicks without seeing neither the black holes nor the transient possibly
associated with their formation. Binarity also has a large impact on the
predicted population of supernova sub-types, including hydrogen-rich type II
supernovae, with a significant fraction of hydrogen-rich stars at explosions
being either merger products or accretors.
