  We study correlation functions and Bethe Salpeter amplitudes for the scalar,
the pseudoscalar and the tensor glueballs using an instanton-based model of the
QCD vacuum. We consider both the pure gauge case and the situation for real QCD
with two light quark flavors. We show that instantons lead to a strong
modification of the correlation functions as compared to their perturbative
behavior. In particular, we find a strong attractive force in the
$J^{CP}=0^{++}$ channel and repulsion in the $0^{+-}$ channel. Due to the
strong classical field of the instantons, these effects are much larger than
the spin splittings observed in mesons made of quarks. The resulting masses,
coupling constants and wave functions appear to be in agreement with lattice
gauge simulations.
