Аннотация:If a supernova remnant (SNR) which has evolved to its radiative stage is advancing through interstellar gas of density 0.2-1 cu cm, then its optical emission should surpass the night-sky background so long as the SNR expansion velocity v(s) remains above 50-60 km/sec. Radiation will emanate not only from the thin, quasi-uniform blast-wave shell but also from the system of filaments which Pikel'ner (1954 and 1973) believed would form when parts of the shock front are refracted by different interstellar irregularities and intersect. It is possible to estimate v(s) either from the Doppler-shifted shell emission lines or from the velocities of the faintest filaments, as these will approach v(s) asymptotically.