Heavy quarkonia and new hadrons with two heavy quarks

We give a pedagogical introduction to heavy quarkonia -- bound states of a heavy quark and its antiquark (e. g. , charmonium cc, bottomonium bb) -- as well as to the exotic hadrons containing two heavy quarks that have been discovered since 2003. The review covers the foundational discoveries (J/ψ and Υ), basic properties, spectroscopy interpreted via potential models, production mechanisms at colliders, and decay modes. A significant focus is placed on the so-called ``XYZ" states -- particles like the X (3872), Y (4260/4230), Zc (3900), and Pc -- whose properties defy conventional quark model expectations. These states, considered candidates for hybrids, multi-quark states, hadronic molecules, or hadroquarkonia, provide unprecedented probes of non-perturbative QCD and challenge our understanding of quark confinement and hadron formation. The chapter summarizes the current experimental landscape and highlights key open questions driving future research in hadron spectroscopy.