  Relativistic jets carry energy and particles from compact to very large
scales compared with their initial radius. This is possible due to their
remarkable collimation despite their intrinsic unstable nature. In this
contribution, I review the state-of-the-art of our knowledge on instabilities
growing in those jets and several stabilising mechanisms that may give an
answer to the question of the stability of jets. In particular, during the last
years we have learned that the limit imposed by the speed of light sets a
maximum amplitude to the instabilities, contrary to the case of classical jets.
On top of this stabilising mechanism, the fast growth of unstable modes with
small wavelengths prevents the total disruption and entrainment of jets. I also
review several non-linear processes that can have an effect on the collimation
of extragalactic and microquasar jets. Within those, I remark possible causes
for the decollimation and decelleration of FRI jets, as opposed to the
collimated FRII's. Finally, I give a summary of the main reasons why jets can
propagate through such long distances.
