**6. Concluding Remarks**

In this review, we attempted at summarizing the historical development of the current ideas on the processes that shape the structure of relativistic jets in gamma-ray bursts and the main observational consequences of that structure. We put some emphasis on the qualitative physical description of the structure formation and on the imprint of the structure on the main observables that characterize the prompt and afterglow emission of gamma-ray bursts, with the aim of providing first guidance (with minimal use of technicalities and a pedagogical approach) to those who encounter these topics for the first time. Inevitably, the review only covers a fraction of the relevant literature, only scratches the surface of most arguments, and represents only a partial account of the huge scientific effort performed by the community in the last few decades on this topic. For example, we entirely neglected the important topic of the polarization of the emission from a structured jet: fortunately, this is brilliantly covered by another review article [45] in this same Special Issue.

Most of the topics presented here are fields of active research and are evolving at a fast pace, especially in the latest years after GW170817. The upcoming science runs of the ground-based gravitational wave detector network, currently comprising the Laser Interferometer Gravitational-wave Observatory (LIGO, [295]), Advanced Virgo [296] and KAGRA [297], will likely soon yield at least one new binary neutron star and/or black hole-neutron star merger event (e.g., [298–307]), hopefully with an associated jet: this will provide new unique insights on the structure of short GRB jets, on the properties of off-axis

jet emission in general, and on the incidence of jets, which can be used to constrain the progenitor population (e.g., [308]). With new 'golden' events such as GW170817, direct information on the jet structure can be extracted, e.g., through the methodologies discussed at the end of Section 4.2, provided that a detailed, high-cadence, multi-wavelength dataset will be collected. The availability of VLBI observations will greatly enhance the chances to break the inherent degeneracies in the afterglow modeling and hence in the jet structure. Moreover, new events observed at differing viewing angles will be the perfect route to test the quasi-universal structured jet hypothesis. We look forward to learning much more about the structure of gamma-ray burst jets from these and other observations, and from theoretical advances, in the close future.

**Funding:** This research was funded by the Italian Ministry of University and Research through the PRIN grant 20179ZF5KS and by the Italian National Institute for Astrophysics (INAF) through the PRIN grant "Towards the SKA and CTA era: discovery, localisation, and physics of transient sources" (1.05.01.88.06).

**Acknowledgments:** We thank G. Ghisellini for years of fruitful discussions. We thank Yuri Sato for useful comments. We thank the anonymous referees for their comments, which helped in improving the quality and completeness of this review.

**Conflicts of Interest:** The authors declare no conflict of interest.
