If the cavity resonance is instead tuned to the vibrational (i.e., instead of electronic) transitions in the molecule, referred to as the vibrational strong coupling regime, enhancement and suppression of ground state chemical reactions have been experimentally observed. We presented a few recently proposed theoretical explanations of this (largely unresolved) phenomenon in Sec. 5. Within this section, we first introduced a model Hamiltonian in Sec. 5.1 for a single molecule coupled to a cavity radiation mode and showed in Sec. 5.2 why simple one-dimensional classical transition state theory (TST) fails to predict any modification to the chemical reactivity when coupling to the cavity for this model. In Secs. 5.3-5.4 we further developed the model and showed how Grote-Hynes (GH) rate theory (or the multi-dimensional TST), which also treats all degrees of freedom (DOFs) classically, predicts a suppression of chemical reactivity and provides a conceptually simple idea -the so-called solvent caging effect, where the cavity radiation mode acts as a non-Markovian solvent DOF -to explain the cavity-mediated suppression. Within this section, we also showed that approximate quantum corrections to the GH theory tend to depart further from experimental observations while exact quantum dynamics simulations make predictions much closer to experiments and depict similar features in the chemical rate that are sharply peaked at resonance conditions. In Sec. 5.5 we showed that the cavity photonic mode, which was shown to act as a solvent DOF in Sec. 5.4, can also enhance the chemical reactivity when solvent-molecule interactions are weak. Finally, to conclude this section, we described in Sec. 5.6 how the cavity can also modify thermally activated non-adiabatic electron transfer reactions.