Supernova remnants (SNRs) are now established as cosmic particle accelerators through observations of nonthermal emissions from radio to gamma rays in the past decades. In the context of Galactic cosmic-ray origin, one of the key questions is if SNRs are capable of accelerating protons with sufficiently high efficiency to explain the cosmic-ray spectrum below the knee. At least for some SNRs, gamma-ray emissions in the GeV band are solidly attributed to decay of neutral pions, providing long-awaited evidence for proton acceleration. The next and current burning question is if protons are accelerators up to the knee at ~ PeV. We review recent results with particular emphasis on this topic and also discuss future prospects.