  The effects of binaries on population studies of stars and stellar phenomena
have been investigated over the past 3 decades by many research groups. Here we
will focus mainly on the work that has been done recently in Brussels and we
will consider the following topics: the effect of binaries on overall galactic
chemical evolutionary models and on the rates of different types of supernova,
the population of point-like X-ray sources where we distinguish the standard
high mass X-ray binaries and the ULXs, a UFO-scenario for the formation of
WR+OB binaries in dense star systems. Finally we critically discuss the
possible effect of rotation on population studies.
