  Extragalactic jets are a common feature of radio-loud active galaxies. The
nature of the observed jets in relation to the bulk flow is still unclear. In
particular it is not clear whether the observations of parsec-scale jets using
the very long baseline interferometric technique (VLBI) reveal wave-like
structures that develop and propagate along the jet, or trace the jet flow
itself. In this contribution I review the evidence collected during the last
years showing that the ridge-lines of helical radio-jets do not correspond to
observational artifacts. This conclusion was reached by studying a number of
VLBI observations of the radio jet in the quasar S5~0836+710 at different
frequencies and epochs. The ridge-line of the emission in the jet coincides at
all frequencies within the errors. Moreover, small differences between the
ridge-lines as observed at different epochs reveal wave-like motion transversal
to the jet propagation axis. I also discuss similar results, albeit with
different interpretations, obtained by other authors. The current challenge is
to measure the propagation velocities of these waves and to try to characterise
them in terms of simple perturbations or Kelvin-Helmholtz instability, which
would help understanding the physical conditions of the flow where the waves
develop. This problem can only be tackled by high-resolution observations such
as those that can be achieved by the space radio-antenna Radioastron.
