  Since the majority of massive stars are members of binary systems, an
understanding of the intricacies of binary interactions is essential for
understanding the large variety of supernova types and sub-types. I therefore
briefly review the basic elements of binary evolution theory and discuss how
binary interactions affect the presupernova structure of massive stars and the
resulting supernovae.
  SN 1987A was a highly anomalous supernova, almost certainly because of a
previous binary interaction. The most likely scenario at present is that the
progenitor was a member of a massive close binary that experienced dynamical
mass transfer during its second red-supergiant phase and merged completely with
its companion as a consequence. This can naturally explain the three main
anomalies of SN 1987A: the blue color of the progenitor, the chemical anomalies
and the complex triple-ring nebula.
