VLBI at the highest frequencies - AGN studied with micro-arcsecond   resolution
  Compact galactic and extragalactic radio sources can be imaged with an unsurpassed angular resolution of a few ten micro-arcseconds, adopting the observing technique of global millimeter VLBI. Here we present the Global Millimeter VLBI Array (GMVA) and discuss its present performance. For individual and partially archetypical radio sources with prominent VLBI jets (e.g. 3C120, Cygnus A, M87, 3C454.3, NRAO150),we show and discuss new results obtained with the GMVA. The variety of observed effects range from jet propagation and bending, partial fore-ground absorption in the nucleus, and jet component ejection after major flares to new and very small (15-20 Schwarzschild radii) upper limits to the jet base of M87. We also discuss the future development of mm-VLBI at 3mm and towards shorter wavelengths, and make suggestions for possible improvements. 
T.P. Krichbaum, I. Agudo, U. Bach, A. Witzel, J.A. Zensus