Abstract: This seminar aims to show how quarkonium is an extraordinary tool to explore the behaviour of a strongly-coupled theory, in this case the Quantum-chromodynamics, in its transition from the perturbative to the non-perturbative regime. Quarkonium is a family of states with mass between 3 and 10 GeV/c2 usually described as bound states of a heavy quark and its antiquark. In such systems a hierarchy of energy scales naturally emerges: from the soft, non-perturbative scale that controls the spectrum to the hard scale of the annihilations. At each of these scales we are now observing unexpected phenomena, all connected to the emergence of light degrees of freedom inside the heavy meson. In this seminar I will introduce the basic features of quarkonia, review the most important recent progresses in connection with our understanding of QCD in the non-perturbative regime and finally outline the future lines of research in this field, with a focus on the Belle II experiment.