  The recent improvement in VLBI arrays is providing information of the
emission and magnetic field structure of relativistic jets, both extragalactic
and galactic (microquasars), with unprecedented spatial and temporal
resolution. These observations are revealing the importance of the
hydrodynamical processes that govern the jet evolution, which can be studied by
the recently developed time-dependent relativistic hydrodynamical models.
Computation of the non-thermal emission from these hydrodynamical models, and
its comparison with actual sources, is proving as one of the most powerful
tools in the understanding of the physical processes taking place in these
jets. This paper reviews some of the recent observational results, as well as
the numerical models used to interpret them.
