  It has now been known for over a decade that low-mass stars located in
star-forming regions are very frequently members of binary and multiple
systems, even more so than main sequence stars in the solar neighborhood. This
high multiplicity rate has been interpreted as the consequence of the
fragmentation of small molecular cores into a few seed objects that accrete to
their final mass from the remaining material and dynamically evolve into stable
multiple systems, possibly producing a few ejecta in the process. Analyzing the
statistical properties of young multiple systems in a variety of environments
therefore represents a powerful approach to place stringent constraints on star
formation theories. In this contribution, we first review a number of recent
results related to the multiplicity of T Tauri stars. We then present a series
of studies focusing on the multiplicity and properties of optically-undetected,
heavily embedded protostars. These objects are much younger than the previously
studied pre-main sequence stars, and they therefore offer a closer look at the
primordial population of multiple systems. In addition to these observational
avenues, we present new results of a series of numerical simulations that
attempt to reproduce the fragmentation of small molecular cores into multiple
systems, and compare these results to the observations.
