  Long-baseline interferometry at infrared wavelengths allows the innermost
regions around young stars to be observed. These observations directly probe
the location of the dust and gas in the disks. The characteristic sizes of
these regions found are larger than previously thought. These results have
motivated in part a new class of models of the inner disk structure, but the
precise understanding of the origin of these low visibilities is still in
debate. Mid-infrared observations probe disk emission over a larger range of
scales revealing mineralogy gradients in the disk. Recent spectrally resolved
observations allow the dust and gas to be studied separately showing that the
Brackett gamma emission can find its origin either in a wind or in a
magnetosphere and that there is probably no correlation between the location of
the Brackett gamma emission and accretion. In a certain number of cases, the
very high spatial resolution reveals very close companions and can determine
their masses. Overall, these results provide essential information on the
structure and the physical properties of close regions surrounding young stars
especially where planet formation is suspected to occur.
