In the last years, the improvement of performances of high-energy instruments provided a great amount of data relating to Cosmic-Rays. In particular, in the gamma-ray band, we had the first evidence of CR energization at Supernova Remnant shocks. Several models were developed in order to isolate the hadronic component from the leptonic one and to find the first direct proof of CR acceleration in those sources. Because of some spectral features in disagreement with the Diffusive Shock Acceleration theory, however, the role of pre-existing CR re-acceleration was taken into account, finding that not only its contribution is not negligible but also that it could explain some important characteristics of CR particle and gamma-ray spectra