  In this paper we discuss the early phase of radio source evolution as
represented by Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS)
radio sources. Correlations between their spectral peak and angular size
strongly suggest that the spectral turnovers are caused by synchrotron self
absorption, and indicate that young radio sources evolve in a self similar way.
We argue that the evolution of a radio source during its first 10^5 years is
qualitatively very different from that during the rest of its life-time. This
may be caused by the difference in the density gradient of the intra-galactic
medium inside and outside the core-radius of the host galaxy.
