  The study of the internal structure of hadronic jets has become in recent
years a very active area of research in particle physics. Jet substructure
techniques are increasingly used in experimental analyses by the LHC
collaborations, both in the context of searching for new physics and for
Standard Model measurements. On the theory side, the quest for a deeper
understanding of jet substructure algorithms has contributed to a renewed
interest in all-order calculations in QCD. This has resulted in new ideas about
how to design better observables and how to provide a solid theoretical
description for them. In the last years, jet substructure has seen its scope
extended, for example, with an increasing impact in the study of heavy-ion
collisions, or with the exploration of deep-learning techniques. Furthermore,
jet physics is an area in which experimental and theoretical approaches meet
together, where cross-pollination and collaboration between the two communities
often bear the fruits of innovative techniques. Despite the wealth of
literature on this topic, we feel that a comprehensive and, at the same time,
pedagogical introduction to jet substructure is still missing. This makes the
endeavour of approaching the field particularly hard, as newcomers have to
digest an increasing number of substructure algorithms and techniques, too
often characterised by opaque terminology and jargon. Furthermore, while
first-principle calculations in QCD have successfully been applied in order to
understand and characterise the substructure of jets, they often make use of
calculational techniques, such as resummation, which are not the usual textbook
material. Thus, the idea of combining our experience in different aspects of
jet substructure phenomenology to put together this set of lecture notes, which
we hope could help and guide someone who moves their first steps in the physics
of jet substructure.
