Extragalactic jets have always had two characteristics : the presence of knots and the requirement for particle acceleration. Shock fronts provide an explanation for both. However, the knotty appearance is less obvious at kp-scale on very high resolution observations from the VLA and the HST. The evidence for shock fronts is therefore weakened. At the pc-scale (or VLBI scale), a lot of these blobs are moving superluminally and they have been interpreted and modelled as shock fronts in a relativistic jet.