We review the subject of the energy levels of light and heavy quark-antiquark bound states in potential models, with emphasis on potentials which are motivated from quantum chromodynamics. In addition to pointing out the successes of potential models, we discuss problems with the potential approach, including uncertainties in the form of the potential, its spin dependence, its transformation properties, and its nondiagonal terms. We consider two-body wave equations with both nonrelativistic and relativistic kinematics, pointing out that in the latter case, a wide variety of different wave equations have been used in the literature.