  In the 1960s, the remnants of supernova explosions (SNRs) were indicated as a
possible source of galactic cosmic rays through the Diffusive Shock
Acceleration (DSA) mechanism. Since then, the observation of gamma-ray emission
from relativistic ions in these objects has been one of the main goals of
high-energy astrophysics. A few dozen SNRs have been detected at GeV and TeV
photon energies in the last two decades. However, these observations have shown
a complex phenomenology that is not easy to reduce to the standard paradigm
based on DSA acceleration. Although the understanding of these objects has
greatly increased, and their nature as efficient electron and proton
accelerators has been observed, it remains to be clarified whether these
objects are the main contributors to galactic cosmic rays. Here, we review the
observations of {\gamma}-ray emission from SNRs and the perspectives for the
future.
