  Intensive VLBI monitoring programs of jets in AGN are showing the existence
of intricate emission patterns, such as upstream motions or slow moving and
quasi-stationary componentes trailing superluminal features. Relativistic
hydrodynamic and emission simulations of jets are in very good agreement with
these observations, proving as a powerful tool for the understanding of the
physical processes taking place in the jets of AGN, microquasars and GRBs.
These simulations show that the variability of the jet emission is the result
of a complex combination of phase motions, viewing angle selection effects, and
non-linear interactions between perturbations and the underlying jet and/or
ambient medium. Both observations and simulations suggest that shock-in-jet
models may be an overly simplistic idealization when interpreting the emission
structure observed in actual jets.
