Plerions, i.e. supernova remnants resembling the Crab Nebula, are characterized at radio wavelengths by having: 1) a centrally peaked brightness distribution, 2) flat radio spectrum (α > −0.3, Sν να), 3) high linear polarization, and 4) a highly ordered magnetic field. At higher frequencies (1011–1013 Hz depending upon the age) the spectrum turns over and attains a slope of α ~ −1. In particular, for the Crab Nebula the turnover frequency occurs at νc ~ 3×1012 Hz (cf. Fig. 1). Moreover, the optical spectrum displays a slope of −0.9 and the radiation is linearly polarized. Nonthermal emission is also detected in the X-ray and γ-ray domains (Fig.1). Similar spectral characteristics are found for other bona fide plerions. As discussed by Weiler and Panagia (1980), the plerion phenomenon is determined by the presence of a highly energetic and active central object (a fast spinning neutron star) which is able to both accelerate and inject continuously relativistic electrons into the remnant with a typically flat energy distribution roughly proportional to E−1.