Shocked atomic and molecular gas in supernova remnants

Hi 21 cm line emission from fast-moving atomic gas has been detected toward 27 SNRs. The HI gas was usually thought to be part of an expanding Hi shell, as expected for an old supernova remnant in a uniform medium. But recent studies show that a good (;greater than or similar to 20-30%) fraction of these remnants are interacting with molecular clouds and that the fast-moving Hi gas in some of these remnants is likely to have been part of the molecular cloud at first. 1 review the results of recent studies on SNRs interacting with molecular clouds and discuss the implications of the scarcity of supernova remnants with expanding Hi shells.