  The properties of multiple stellar systems have long provided important
empirical constraints for star formation theories, enabling (along with several
other lines of evidence) a concrete, qualitative picture of the birth and early
evolution of normal stars. At very low masses (VLM; M <~ 0.1 M_sun), down to
and below the hydrogen burning minimum mass, our understanding of formation
processes is not as clear, with several competing theories now under
consideration. One means of testing these theories is through the empirical
characterization of VLM multiple systems. Here, we review the results of
various VLM multiplicity studies to date. These systems can be generally
characterized as closely separated (93% have projected separations Delta < 20
AU) and near equal-mass (77% have M_2/M_1 >= 0.8) occurring infrequently
(perhaps 10-30%). Both the frequency and maximum separation of stellar and
brown dwarf binaries steadily decrease for lower system masses, suggesting that
VLM binary formation and/or evolution may be a mass-dependent process. There is
evidence for a fairly rapid decline in the number of loosely-bound systems
below ~0.3 M_sun, corresponding to a factor of 10-20 increase in the minimum
binding energy of VLM binaries as compared to more massive stellar binaries.
This wide-separation ``desert'' is present among both field (~1-5 Gyr) and
older (> 100 Myr) cluster systems, while the youngest (<~10 Myr) VLM binaries,
particularly those in nearby, low-density star forming regions, appear to have
somewhat different systemic properties. We compare these empirical trends to
predictions laid out by current formation theories, and outline future
observational studies needed to probe the full parameter space of the lowest
mass multiple systems.
