Introduction

Circumstellar interaction has turned out to be of crucial importance for the interpretation of observations of supernovae at both early and late stages. Much of the progress in this field is a result, of the combination of radio, optical, UV and X-ray observations. Here I review the basic evidence for circumstellar interaction, some of the most important physical processes, and specific examples at both early and late, stages in the supernova evolution. For a complementary review see especially the excellent review by Chevalier (1990).

Observational evidence for circumstellar interaction

The first evidence that circumstellar interaction is important for supernovae came from observations of SN 1979C. UV observations during the first few weeks showed a number of emission lines, interpreted as a result of circumstellar interaction (§4.3). Unambiguous evidence for circumstellar interaction came from radio observations more than a year later, showing a wavelength-dependent turn-on of the radio emission (Sramek & Weiler 1990; Van Dyk, this volume). Emission was first seen at short wavelengths, and later at longer. This behavior is interpreted as a result of decreasing free-free absorption by the ionized gas in a circumstellar medium around the supernova (Chevalier 1982b).