Global VLBI imaging at millimeter and sub-millimeter wavelengths overcomes the opacity barrier of synchrotron self-absorption in Active Galactic Nuclei (AGN) and opens the direct view into sub-pc scale regions not accessible before. Since AGN variability is more pronounced at short millimeter wavelength, mm-VLBI can reveal structural changes in very early stages after outbursts. When combined with observations at longer wavelength, global 3 mm and 1 mm VLBI adds very detailed information on the source structure. This helps to determine fundamental physical properties at the jet base, and in the vicinity of super-massive black holes at the center of AGN. Here we present new results from multi-frequency mm-VLBI imaging of OJ 287 during a major outburst. We also report on a successful 1.3 mm VLBI experiment with the APEX telescope in Chile. This observation sets a new world record in angular resolution. It also opens the path towards future mm-VLBI with ALMA, which aims at the mapping of the black hole event horizon in nearby galaxies, and the study of the roots of jets in AGN.