It is well known that M. Gell-Mann, introducing quarks in 1964 to describe
the known mesons and baryons, hinted at the existence of further qqqΛβqΛβ mesons (tetraquarks) and qqqqqΛβ baryons (pentaquarks). In 1977, R.
Jaffe proposed a model of the lightest scalar mesons as diquark-antidiquark
pairs and A. de Rujula, H. Georgi and S. Glashow coined the term hadron
molecules, to describe possible hadrons made by meson-antimeson pairs bound by
the familiar nuclear forces, also an overall tetraquark system. The two
alternative pictures have been employed to interpret the unexpected hadron
discovered by Belle in 2003, the X(3872), confirmed by BaBar and seen in many
other High Energy experiments. Since then, a wealth of Exotic Hadrons have been
discovered, mesons and baryons that cannot be described by the classical
Gell-Mann, qqΛβ and qqq, configurations, opening a new chapter of Hadron
Spectroscopy.Comment: Lectures for the school "Frontiers in Nuclear and Hadronic Physics
2022", held at the Galileo Galilei Institute, Florence (Italy), February
21-25, 202