In this chapter, we present a brief review of the two-body problem, as it enters into the non-relativistic quark model of mesons. Experts can easily skip this chapter and read instead more advanced reviews on quarkonium [17, 18, 19, 20]. Note, however, that we present some mathematical results on the level ordering, level spacing, and mass dependence of the energy, which will be useful for the generalization to the three-body case. Important concepts will be introduced for nonexperts: Jacobi variables, scaling laws, numerical solutions, i.e., the basic tools for few-body calculations.