  We review recent advances in our understanding of the innermost regions of
the circumstellar environment around young stars, made possible by the
technique of long baseline interferometry at infrared wavelengths.
Near-infrared observations directly probe the location of the hottest dust. The
characteristic sizes found are much larger than previously thought, and
strongly correlate with the luminosity of the central young stars. This
relation has motivated in part a new class of models of the inner disk
structure. The first mid-infrared observations have probed disk emission over a
larger range of scales, and spectrally resolved interferometry has for the
first time revealed mineralogy gradients in the disk. These new measurements
provide crucial information on the structure and physical properties of young
circumstellar disks, as initial conditions for planet formation.
