Theory of resonances

We give a pedagogical introduction to the theory of resonances, focusing specifically on the spectrum of excited strongly interacting particles. After providing historical context starting from the atomic theory, we summarize the status of theoretical and experimental research. We discuss then the methodology to determine universal resonance parameter through the analytical properties of the transition amplitudes. For this, the main aspects of the $S$-matrix theory are introduced including some explicit calculations in simplified cases. In the last section, we summarize several frequently used amplitude types also making a connection to Effective Field Theories, as well as provide first glimpse into some more advanced applications to further resonance properties.