Compact Steep-Spectrum (CSS) and GHz-Peaked Spectrum (GPS) radio sources are intrinsically small objects (Fanti et al. 1990) with high frequency steep spectra (α > 0.5 with S∝ v –α), found at moderate or high redshifts (z>0.2 but many objects have z>1.5). Their characteristics have been interpreted in terms of youth (e.g. Fanti et al. 1995) or “frustration” (e.g. van Breugel et al. 1984). Their radio spectra turn over at tens or a few hundreds of MHz (CSS) or at higher frequencies (GPS), interpreted as due to synchrotron self-absorption. They represent a significant fraction of flux limited catalogues (15–30 %, depending on the frequency).