  Recent observations in the TeV band challenge the simplest models developed
to describe the overall emission of blazars and radiogalaxies. In particular,
the observation of variable TeV emission from M87 and the fast variability
shown by PKS 2155-304 challenge the standard framework. We discuss how the
existence of a radial structure in the sub-pc scale jet, with faster a
component ("spine" or "needles") embedded in a slower layer can explain the
basic phenomenology of these sources.

Astrophysics