The radio emission from supernovae has been modeled as synchrotron emission from the interaction between the supernova and a presupernova stellar wind. The emission shows a late power-law decline and early rise due to a low-frequency absorption process, which, in some cases (SN 1979C and SN 1980K), is well modeled as free-free absorption by the external wind. However, the flux rises in many radio supernovae (e.g., SN 1987A and Type Ib and Type Ic supernovae) have not been sufficiently well observed to define the absorption mechanism. The assumption of synchrotron self-absorption yields an approximate radius of the emission region at the time of peak flux. If another mechanism is dominant, the radius must be even larger. The large radii implied for SN 1987A and the observed Type Ib and Type Ic supernovae make external free-free absorption unlikely, given what else is known about these systems, so that synchrotron self-absorption is the probable absorption mechanism. Synchrotron self-absorption may play a role in the radio emission from SN 1981K (type unknown) and SN 1978K (Type IIn). Model supernova light curves with synchrotron self-absorption are presented. A combination of synchrotron self-absorption and external free-free absorption may be relevant to SN 1993J. The radii implied for the Type II supernovae SN 1979C and SN 1980K in the synchrotron self-absorption model are smaller than the radii expected for circumstellar interaction and are thus consistent with the free-free absorption mechanism.