**3. Open Questions**

As predicted by the basic standard model presented in the previous section, the afterglow emission is the result of particle acceleration and radiative cooling occurring in two different regions: the forward and the reverse shock. The temporal and spectral behavior of the two emission components can be inferred after the jet/blastwave dynamics, acceleration mechanisms and the radiation processes are modeled (Section 2). The general agreement between model predictions and observations convincingly proves that the long-lasting radio-to-GeV radiation is indeed produced in interactions between the ejecta and the external medium. Moreover, the radiative mechanisms involved and the nature of emitting particles are well established, with synchrotron (and possibly SSC) from the accelerated electrons (either at the forward or reverse shock) being the source of the detected radiation.

Despite the general success of the external shock scenario, there are several, longstanding open issues which represent a serious challenge for our present understanding of the afterglow emission and the GRB phenomenon in general. Moreover, even when observations seem to be in qualitative agreement with predictions, the extraction of the model parameters (which would give important feedbacks on our understanding of particle acceleration and GRB environments) is limited by the large degeneracy among parameters and lack of solid inputs from theoretical considerations.

Afterglow emission studies have not experienced relevant progresses in the last years, with observations and techniques that are the same since the launch of the Swift satellite. The recent discovery of TeV radiation from GRBs is opening the possibility to renovate and boost afterglow studies, with major impacts on the general understanding of GRB sources.

In this section, we list and comment on those aspects still lacking a clear explanation, and in particular we selected topics which might largely benefit from observations and detections in the VHE regime.
