  We discuss the present performance and the future perspectives of VLBI in the
3 mm to 0.85 mm observing bands (so called mm-VLBI). The availability of new
telescopes and the recent technical development towards larger observing
bandwidth and higher data-rates now allow to image with 3mm-VLBI hundreds of
sources with high dynamic range. As an example we show new images of the jets
of Cygnus A. At 1.3 mm, pilot VLBI studies have proven detectability of the
brightest AGN, and the existence of ultra-compact regions therein. In the next
few years global VLBI imaging will be established also at 1.3 mm and 0.85 mm
wavelength. With an angular resolution in the 10-20 micro-arcsecond range,
future 1.3 mm- and 0.8 mm VLBI will be an extraordinarily powerful astronomical
observing method, allowing to image the enigmatic `central engines' and the
foot-points of AGN-jets in greater detail than ever possible before. A
sufficiently large number of telescopes is a prerequisite for global aperture
synthesis imaging. Therefore a strong effort is needed to make more telescopes
available for VLBI at short millimeter and sub-millimeter wavelengths. In this
context, the further VLBI upgrade of both IRAM telescopes and the outfit of the
APEX telescope in Chile, in preparation for later mm-/sub-mm VLBI with ALMA, is
of high scientific importance. With a sufficiently large mm-VLBI network, the
micro-arcsecond scale imaging of the post-Newtonian emission zone around the
event horizon/ergosphere of nearby super-massive Black Holes (such as e.g. Sgr
A*, M87, ...) should become possible within the next few years.
