The development of instabilities leading to the formation of internal shocks is expected in the relativistic outflows of both gamma-ray bursts and blazars. The shocks heat the expanding ejecta, generate a tangled magnetic field and accelerate leptons to relativistic energies. While this scenario has been largely considered for the origin of the spectrum and the fast variability in gamma-ray bursts, here we consider it in the contest of relativistic jets of blazars. We calculate the expected spectra, light curves and time correlations between emission at different wavelengths. The dynamical evolution of the wind explains the minimum distance for dissipation (~10^{17} cm) to avoid $gamma$--$gamma$ collisions and the low radiative efficiency required to transport most of the kinetic energy to the extended radio structures. The internal shock model allows to follow the evolution of changes, both dynamical and radiative, along the entire jet, from the inner part, where the jet becomes radiative and emits at high energies ($gamma$-jet), to the parsec scale, where the emission is mostly in the radio band (radio-jet). We have produced some animations that can be found at http://www.merate.mi.astro.it/~lazzati/3C279/, in which the temporal and spectral informations are shown together.
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