  (abridged) Relativistic jets, formed in the vicinity of central supermassive
black holes in AGN, show ample evidence connecting them to physical conditions
in the accretion disc and broad-line region. The jets are responsible for a
large fraction of non-thermal continuum emission (particularly during powerful
flares), which makes understanding their physics an important aspect of studies
of blazars characterised by profound flaring activity arising from extremely
compact regions. Imaging and polarimetry of radio emission on milliarcsecond
scales provided by very long baseline interferometry (VLBI) offers a range of
possibilities for studying ultra-compact regions in relativistic jets and
relating them to main manifestations of the blazar activity in AGN.
Simultaneous monitoring of optical/high energy variability and evolution of
parsec-scale radio structures yields arguably the most detailed picture of the
relation between acceleration and propagation of relativistic flows and
non-thermal continuum generation in blazars. These effects are reviewed and
discussed in the context of deriving accurate and self-consistent models for
central regions of blazars.

Cosmology and Nongalactic Astrophysics;High Energy Astrophysical Phenomena