  We present recent results in semileptonic and non-leptonic exclusive $B_c$
decays to charmonium states both in $S$-wave, $J/\psi$ and $\eta_c$, and in
$P$-wave, $\chi_{cJ}$ and $h_c$. The analysis, based on the heavy quark spin
symmetry (HQSS), produces relations among form factors that parametrize the
hadronic matrix elements in the amplitudes of the decays. These relations are
helpful to control the hadronic uncertainty affecting these processes.
Furthermore, $B_c$ decays allow us to get hints on the structure of states like
$\chi_{c1}(3872)$, whose exotic or ordinary charmonium nature is debated.
