In this paper, we review the roles of collective modes in nuclear reactions. We emphaize the strong couplings of various collective states with the monopole and quadrupole motions. In inelastic excitation, these couplings can be seen as an important source of anharmonicity in the multiphonon spectrum. In fusion reaction, the breathing and quadrupole motions strongly affects the oscillation of protons against neutrons. Finally, the modification of the collective properties induced by a large amplitude dilution might be the origin of the nuclear multifragmentation, directly related to the nuclear liquid-gas phase transition. In the three cases we derive a coupling matrix element which appears to be in good agreement.