  In this thesis we study two processes in which instantons may play an
important role: the decays of charmonium states $\eta_c$, $\chi_c$ and glueball
and the OZI violation in channels relevant to the proton spin decomposition
puzzle. We also present an algorithm for computation of integrals over compact
groups and exemplify it on integrals over SU(N) fundamental and adjoint
representations.
