I will review the current understanding (or lack of thereof) about the nature of several observed “exotic” resonances. After introducing the experimental puzzles, I will describe the two main theoretical interpretations: the compact tetraquarks and the loosely bound hadronic molecules. I will put emphasis on the successes and failures of both models. Finally, I will conclude with a (partial) list of possible observables which might be key to discriminate between the two possibilities in the future.
Riccardo Rattazzi and Angelo Esposito