Nonfactorization in B and D decays

We discuss the role of nonfactorized contributions in $B \rightarrow \psi + {K}^{*}$ and charmed meson decays. We demonstrate, using ${D}^{+}_{S} \rightarrow \phi \pi$ as a model, how nonfactorization, annihilation and final state interactions can be built into effective and unitarized ${a}_{1}$ and ${a}_{2}$.