What is a resonance? And why does it matter?

The resonance phenomenon is of central importance in many areas of physics, with particular significance in the study of nuclear structure and reactions. Starting from the classical framework of damped driven oscillations, this text introduces and analyzes quantum-mechanical resonances in a pedagogical and systematic fashion, with emphasis on applications in nuclear physics. Building on the formal theory of resonances, the text elucidates the relationship between experimental observations, phenomenological insights, and computational methods used to characterize and describe resonant states. The discussion encompasses the diverse manifestations of nuclear resonances, ranging from few- to many-body systems, all the way to collective phenomena and to exotic systems that appear near the limits of nuclear stability. References to the relevant literature are provided to assist readers who wish to explore specific topics in more depth.