  Observations from optical to centimeter wavelengths have demonstrated that
multiple systems of two or more bodies is the norm at all stellar evolutionary
stages. Multiple systems are widely agreed to result from the collapse and
fragmentation of cloud cores, despite the inhibiting influence of magnetic
fields. Surveys of Class 0 protostars with mm interferometers have revealed a
very high multiplicity frequency of about 2/3, even though there are
observational difficulties in resolving close protobinaries, thus supporting
the possibility that all stars could be born in multiple systems. Near-infrared
adaptive optics observations of Class I protostars show a lower binary
frequency relative to the Class 0 phase, a declining trend that continues
through the Class II/III stages to the field population. This loss of
companions is a natural consequence of dynamical interplay in small multiple
systems, leading to ejection of members. We discuss observational consequences
of this dynamical evolution, and its influence on circumstellar disks, and we
review the evolution of circumbinary disks and their role in defining binary
mass ratios. Special attention is paid to eclipsing PMS binaries, which allow
for observational tests of evolutionary models of early stellar evolution. Many
stars are born in clusters and small groups, and we discuss how interactions in
dense stellar environments can significantly alter the distribution of binary
separations through dissolution of wider binaries. The binaries and multiples
we find in the field are the survivors of these internal and external
destructive processes, and we provide a detailed overview of the multiplicity
statistics of the field, which form a boundary condition for all models of
binary evolution. Finally we discuss various formation mechanisms for massive
binaries, and the properties of massive trapezia.
