Heavy quarks have been instrumental for progress in our exploration of strong interactions. Quarkonium in particular, a heavy quark-antiquark nonrelativistic bound state, has been at the root of several revolutions. Quarkonium is endowed with a pattern of separated energy scales qualifying it as special probe of complex environments. Its multiscale nature has made a description in Quantum Field Theory particularly difficult up to the advent of Nonrelativistic Effective Field Theories. We will focus on systems made by two or more heavy quarks. After considering some historical approaches based on the potential models and the Wilson loop approach, we will introduce the contemporary nonrelativistic effective field theory descriptions, in particular potential Nonrelativistic QCD which entails the Schoedinger equation as zero order problem, define the potentials as matching coefficients and allows systematic calculations of the physical properties. The effective field theory allows us to explore quarkonium properties in the realm of QCD. In particular it allows us to make calculations with unprecedented precision when high order perturbative calculations are possible and to systematically factorize short from long range contributions where observables are sensitive to the nonperturbative dynamics of QCD. Such effective field theory treatment can be extended at finite temperature and in presence of gluonic and light quark excitations. We will show that in this novel theoretical framework, quarkonium can play a crucial role for a number of problems at the frontier of our research, from the investigation of the confinement dynamics in strong interactions to the study of deconfinement and the phase diagram of nuclear matter, to the precise determination of Standard Model parameters up to the emergence of exotics X Y Z states of an unprecedented nature.

44 pages, 13 figures, version to appear in the "Handbook of Nuclear Physics", Springer, 2022, edited by I. Tanihata, H. Toki and T. Kajino, Section 'Quark Nuclear Physics', Section Editor H. Suganuma, references added