  Some extragalactic SNRs are more than two orders of magnitude more luminous
than the young Galactic SNR Cas A. These SNRs are called super-luminous or
ultra-luminous SNRs. Their high luminosities can be caused by chance
superpositions of multiple objects, interactions with a very dense environment,
or unusually powerful supernova explosions. Four super-luminous SNRs are known:
one in NGC 4449, one in NGC 6946, and two in M101. The two remnants in M101,
NGC 5471B and MF83, are recently suggested to be "hypernova remnants" possibly
connected to the GRBs. We have obtained new or archival HST WFPC2 images and
new high-dispersion echelle spectra of these super-luminous SNRs, in order to
examine their stellar and interstellar environments and to analyze their
energetics. We discuss the physical nature of these four SNRs, with a special
emphasis on the two ``hypernova remnants'' in M101.
