  Over the past few decades, our knowledge of jets produced by active galactic
nuclei (AGN) has greatly progressed thanks to the development of
very-long-baseline interferometry (VLBI). Nevertheless, the crucial mechanisms
involved in the formation of the plasma flow, as well as those driving its
exceptional radiative output up to TeV energies, remain to be clarified. Most
likely, these physical processes take place at short separations from the
supermassive black hole, on scales which are inaccessible to VLBI observations
at centimeter wavelengths. Due to their high synchrotron opacity, the dense and
highly magnetized regions in the vicinity of the central engine can only be
penetrated when observing at shorter wavelengths, in the millimeter and
sub-millimeter regimes. While this was recognized already in the early days of
VLBI, it was not until the very recent years that sensitive VLBI imaging at
high frequencies has become possible. Ongoing technical development and wide
band observing now provide adequate imaging fidelity to carry out more detailed
analyses.
  In this article we overview some open questions concerning the physics of AGN
jets, and we discuss the impact of mm-VLBI studies. Among the rich set of
results produced so far in this frequency regime, we particularly focus on
studies performed at 43 GHz (7 mm) and at 86 GHz (3 mm). Some of the first
findings at 230 GHz (1 mm) obtained with the Event Horizon Telescope are also
presented.
