  Using a combination of ground-based and HST imaging, we have constructed a
catalog of 179 supernova remnants (SNRs) and SNR candidates in the nearby
spiral galaxy M51. Follow-up spectroscopy of 66 of the candidates confirms 61
of these as SNRs, and suggests that the vast majority of the unobserved objects
are SNRs as well. A total of 55 of the candidates are coincident with (mostly
soft) X-ray sources identified in deep Chandra observations of M51; searching
the positions of other soft X-ray sources resulted in several additional
possible optical candidates. There are 16 objects in the catalog coincident
with known radio sources. None of the sources with spectra shows the high
velocities (>500 km/s) characteristic of young, ejecta-dominated SNRs like Cas
A; instead, most if not all appear to be middle-aged SNRs. The general
properties of the SNRs, size distribution and spectral characteristics,
resemble those in other nearby spiral galaxies, notably M33, M83, and NGC6946,
where similar samples exist. However, the spectroscopically observed [N
II]:H{\alpha} ratios appear to be significantly higher than in any of these
other galaxies. Although we have explored various ideas to explain the high
ratios in M51, none of the explanations appears to be satisfactory.
