  Observational advances over the last decade have enabled high-resolution,
interferometric studies of forming multiple systems, statistical surveys of
multiplicity in star-forming regions, and new insights into disk evolution and
planetary architectures in these systems. In this review, we compile the
results of observational and theoretical studies of stellar multiplicity. We
summarize the population statistics spanning system evolution from the
protostellar phase through the main-sequence phase and evaluate the influence
of the local environment. In short, most stars are born in multiple stellar
systems, most main sequence stars are members of multiple systems, but most
star systems are single. We describe current models for the origin of stellar
multiplicity and review the landscape of numerical simulations and assess their
consistency with observations. We review the properties of disks and discuss
the impact of multiplicity on planet formation and system architectures.
Finally, we summarize open questions and discuss the technical requirements for
future observational and theoretical progress.
