SNR can generally be recognized as extended sources of continuum radio emission with non-thermal spectra located near the galactic plane. The emission is synchrotron radiation from relativistic electrons which have either been accelerated or trapped in the expanding shell and its associated shocks. Early lists of remnants (e.g. Milne 1970) were culled from general catalogs of radio sources and confirmed by several other kinds of evidence including the presence of shell structure, significant polarization, lack of recombination line emission, strong optical [S II] lines, and soft x-ray emission. While a few efforts to detect more data on faint remnants are continuing (e.g. Bonsignori and Tomasi 1979; Reich and Braunsfurth 1981), about 150 SNR are now known in the Milky Way and most studies have turned to detailed investigation of specific objects to determine their energy sources, emission mechanisms, and interactions with their surroundings. These studies have shown that while most remnants fall within two general categories, standard shell and filled center, there is no unique evolution within a class and irregularities in the local interstellar medium dominate any statistical properties of individual remnants. A few objects, in particular Cas A and CTB80, do not fit within either category.