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 \approx 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 Öpik law with a break point around P \approx 10 d. 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 \leq q = M _ { 2 } / M _ { 1 } \leq 1.0 .