  Circumstellar disks around young stars are the birthsites of planets. It is
thus fundamental to study the disks in which they form, their structure and the
physical conditions therein. The first astronomical unit is of great interest
because this is where the terrestrial-planets form and the angular momentum is
controled via massloss through winds/jets. With its milli-arcsecond resolution,
optical interferometry is the only technic able to spatially resolve the first
few astronomical units of the disk. In this review, we will present a broad
overview of studies of young stellar objects with interferometry, and discuss
prospects for the future.
