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Although the properties and evolution of supernova remnants (SNRs) are generally understood, there are important questions concerning the interaction of SNRs with the interstellar medium. These include the role of SNRs in regulating the relativistic gas (cosmic rays) in galaxies and the degree to which the ISM affects the evolution of SNRs. Statistically significant samples of SNRs observed at several wavelengths have the potential for yielding valuable insight into these questions. To this end, we are carrying out a search for SNRs in the galaxy M33 at optical, radio and X-ray wavelengths. M33 is ideally suited for a study of this nature. Remnants will all be at essentially the same distance and, because M33 is nearly face on, the effects due to interstellar absorption are reduced. Furthermore, M33 is a spiral galaxy, allowing for comparisons with the Milky Way.
We have undertaken new radio, X-ray and optical observations of M33, and, here, we present the sample of radio selected and optically confirmed SNRs and discuss some of the results. We have identified $\approx 100$ non-thermal radio sources within $20\arcmin$ of the center of M33. Many of these sources are likely to be SNRs, and we have made followup spectroscopic observations of these candidates with the MMT and have found many to be associated with shock-heated gas. This radio-selected sample has the advantage over previous optical samples of M33 in that it can detect remnants in highly optically confused regions. As an example, we have identified a SNR located in the giant HII region NGC-592 in M33 (Gordon et al. , 1993 {\em Ap. J.}, in press). This is a particularly interesting HII region because a soft X-ray source is located in it. We found a knot of non-thermal radio emission at the site of the X-ray source and detected shock-heated gas at optical wavelengths thus showing that the X-ray emission is associated with an embedded SNR.
