2 00 8 Towards millimeter VLBI

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 for mm-VLBI 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. 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 wavelength. With an angular resolution in the 10− 20 μ mas range, future 1.3 mmand 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*, M 87, ...) should become possible within the next few years.