  We review the role of rotation in massive close binary systems. Rotation has
been advocated as an essential ingredient in massive single star models.
However, rotation clearly is most important in massive binaries where one star
accretes matter from a close companion, as the resulting spin-up drives the
accretor towards critical rotation. Here, we explore our understanding of this
process, and its observable consequences. When accounting for these
consequences, the question remains whether rotational effects in massive single
stars are still needed to explain the observations.
