  Due to relativistic bulk motion, the structure and orientation of gamma-ray
burst jets have a fundamental role in determining how they appear. The recent
discovery of the GW170817 binary neutron star merger and the associated GRB
boosted the interest in the modelling and search of signatures of the presence
of a (possibly quasi-universal) jet structure in long and short GRBs. In this
review, following a pedagogical approach, we summarize the history of GRB jet
structure research over the last two decades, from the inception of the idea of
a universal jet structure to the current understanding of the complex processes
that shape the structure, that involve the central engine that powers the jet
and the interaction of the latter with the progenitor vestige. We put some
emphasis on the observable imprints of jet structure on prompt and afterglow
emission and on the luminosity function, favoring intuitive reasoning over
technical explanations.
