  The formation of massive stars is one of the major unsolved problems in
stellar astrophysics. However, only few if any of these are found as single
stars, on average massive stars have more than one companion. Many of them are
born in dense stellar clusters and several clusters have an excess of massive
short-period spectroscopic binaries, with severe implication for binary-related
stellar evolution including mergers, and also for the origin of massive runaway
stars. The multiplicity of massive stars seems to increase with increasing
primary mass and with increasing density of young star clusters. These
observations suggest that massive binary and multiple systems originate mainly
from dynamical gravitational interactions and accretion-induced protostellar
collisions in dense clusters. If true, the binary properties of massive stars
in less dense OB associations should be less extreme. This prediction should be
tested by future observations.The paper reviews both the latest observations
and theoretical ideas related to the origin of massive binaries. It concludes
with a speculation on how the binary properties might change with metallicity
(e.g. LMC/SMC).
