[en] Heavy quarkonium is perhaps the best QCD laboratory in existence. Much of what we presently know about the fundamental interactions in nature has resulted from the study of two-body bound states. The hydrogen atom, the deuteron and positronium have provided valuable mechanisms for studying quantum mechanics, the theory of nuclear forces and QED. In the same way, one might hope that the study of two-body bound state systems built from heavy quarks will lead to a detailed understanding of at least some of the fundamental properties of strong interactions. While it is possible to gain some qualitative insights into strong interactions by studying bound states built from light quarks, it is not possible to reach more precise conclusions because of the additional complexities introduced by large relativistic corrections and long range confinement. Charmonium, c anti c bound states, has provided us with the first QCD laboratory where the system could be considered to be at least approximately non-relativistic while at the same time lending itself to perturbative calculations. The substantial mass of the charmed quark (m_c ∼ 1.5 GeV) corresponds to bound states sufficiently small to begin to probe the asymptotically free component of the strong interaction. The study of c anti c as well as b anti b bound states has greatly enhanced our understanding of the strong interaction between elementary particles. The scope of this article will be to review what has been learned to date, particularly on the experimental front, as well as to look at what remains to be done