  After decades of experimental projects and fast-paced technical advances,
optical / infrared (O/IR) interferometry has seen a revolution in the last
years. The GRAVITY instrument at the VLTI with four 8 meter telescopes reaches
thousand times fainter objects than possible with earlier interferometers, and
the CHARA array routinely offers up to 330 meter baselines and
aperture-synthesis with six 1 meter telescopes. The observed objects are
fainter than 19 magnitude, the images have sub-milliarcsecond resolution, and
the astrometry reaches micro-arcsecond precision. We give an overview of
breakthrough results from the past 15 years in O/IR interferometry on the
Galactic Center, exo-planets and their atmospheres, active galactic nuclei,
young stellar objects, and stellar physics. Following a primer in
interferometry, we summarize the technical and conceptual advances which led to
the boosts in sensitivity, precision, and imaging of modern interferometers.
Single-mode beam combiners now combine all available telescopes of the major
interferometers for imaging, and specialized image reconstruction software
advances over earlier developments for radio interferometry. With a combination
of large telescopes, adaptive optics, fringe-tracking, and especially dual-beam
interferometry, GRAVITY has boosted the sensitivity by many orders of
magnitudes. Another order of magnitude improvement will come from upgrades with
laser guide star adaptive optics. In combination with large separation
fringe-tracking, O/IR interferometry will then provide complete sky coverage
for observations in the Galactic plane, and substantial coverage for
extragalactic targets. VLTI and CHARA will remain unique in the era of upcoming
30-40m extremely large telescopes (ELTs).
