  Binaries are excellent astrophysical laboratories that provide us with direct
measurements of fundamental stellar parameters. Compared to single isolated
star, multiplicity induces new processes, offering the opportunity to confront
our understanding of a broad range of physics under the extreme conditions
found in, and close to, astrophysical objects. In this contribution, we will
discuss the parameter space occupied by massive binaries, and the observational
means to investigate it. We will review the multiplicity fraction of OB stars
within each regime, and in different astrophysical environments. In particular
we will compare the O star spectroscopic binary fraction in nearby open
clusters and we will show that the current data are adequately described by an
homogeneous fraction of f~0.44. We will also summarize our current
understanding of the observed parameter distributions of O+OB spectroscopic
binaries. We will show that the period distribution is overabundant in short
period binaries and that it can be described by a bi-modal Oepik law with a
break point around P~10d. The distribution of the mass-ratios shows no
indication for a twin population of equal mass binaries and seems rather
uniform in the range 0.2< q=M_2/M_1<1.0.
