  New images of young stars are revolutionizing our understanding of planet
formation. ALMA detects large grains in planet-forming disks with few AU scale
resolution and scattered light imaging with extreme adaptive optics systems
reveal small grains suspended on the disks' flared surfaces. Tantalizing
evidence for young exoplanets is emerging from line observations of CO and
H-alpha. In this white paper, we explore how even higher angular resolution can
extend our understanding of the key stages of planet formation, to resolve
accreting circumplanetary disks themselves, and to watch planets forming in
situ for the nearest star-forming regions. We focus on infrared observations
which are sensitive to thermal emission in the terrestrial planet formation
zone and allow access to molecular tracers in warm ro-vibrational states.
Successful planet formation theories will not only be able to explain the
diverse features seen in disks, but will also be consistent with the rich
exoplanet demographics from RV and transit surveys. While we are far from
exhausting ground-based techniques, the ultimate combination of high angular
resolution and high infrared sensitivity can only be achieved through
mid-infrared space interferometry.
