The study of Young Stellar Objects (YSOs) is one of the most exciting topics to be handled by long baseline optical interferometry. The magnitudes of these stars are at the edge of capabilities of current optical interferometers limiting the studies to a few of those but are well within the capability of coming large aperture interferometers like the VLT Interferometer, the Keck
Interferometer, the Large Binocular Telescope or 'OHANA. The milli-arcsecond spatial resolution reached by interferometry allows to probe the very close environment of young stars down to a tenth of an
astronomical unit. In this paper, I review the different aspects of star formation that can be tackled by interferometry: circumstellar
disks, multiplicity, jets, etc. I present recent observations
performed with operational infrared interferometers, IOTA, PTI and ISI, and I show why in the next future we will extend these studies with large aperture interferometers.