Supernova remnants (SNRs) have been regarded for many decades as the sources of Galactic cosmic rays (CRs) up to a few PeV. However, only with the advent of Fermi LAT it has been possible to detect — at least in some SNRs— gamma-rays whose origin is clearly hadronic, namely due to the decay of neutral pions produced by collision between relativistic nuclei and the background plasma. When coupled with observations in other bands (from radio to TeV gamma-rays), Fermi LAT data typically present evidence for CR spectra significantly steeper than the standard prediction of the popular Fermi mechanism, forcing us to rethink our theoretical understanding of efficient particle acceleration at strong shocks. We outline how including the effects a CR-triggered magnetic field amplification it is possible to reconcile non-linear models for diffusive shock acceleration with recent gamma-ray observations, in particular providing a successful application of such a theory to Tycho’s SNR.