We discuss systems containing a heavy quark and a heavy antiquark in
the infinite mass limit of QCD. Studying the limit of equal
velocities for both heavy particles, we formulate an effective theory
approach to heavy quarkonia-like systems. The method is well suited
to processes in which the two heavy quarks annihilate, such as
electromagnetic and strong decays of charmonium and bottomonium and
weak decays of $B_c$.