By providing sensitive sub-arcsecond images and integral field spectroscopy in the 25 – 400 ìm wavelength range, a far-IR interferometer will revolutionize our understanding of planetary system formation, reveal otherwise-undetectable planets through the disk perturbations they induce, and spectroscopically probe the atmospheres of extrasolar giant planets in orbits typical of most of the planets in our solar system. The technical challenges associated with interferometry in the far-IR are greatly relaxed relative to those encountered at shorter wavelengths or when starlight nulling is required. A structurally connected far-IR interferometer with a maximum baseline length of 36 m can resolve the interesting spatial structures in nascent and developed exoplanetary systems and measure exozodiacal emission at a sensitivity level critical to TPF-I mission planning. The Space Infrared Interferometric Telescope was recommended in the Community Plan for Far-IR/Submillimeter Space Astronomy, studied as a Probe-class mission, and estimated to cost $800M. The scientific communities in Europe, Japan, and Canada have also demonstrated a keen interest in far-IR interferometry through mission planning workshops and technology research, suggesting the possibility of an international collaborative effort.