  Baldwin (1982) wrote that \emph{the distribution of sources in the radio
luminosity, $P$, overall physical size, $D$, diagram} could be considered as
\emph{the radio astronomer's $H-R$ diagram}. However, unlike the case of stars,
not only the intrinsic properties of the jets, but also those of the host
galaxy and the intergalactic medium are relevant to explain the evolutionary
tracks of radio radio sources. In this contribution I review the current status
of our understanding of the evolution of radio sources from a theoretical and
numerical perspective, using the $P-D$ diagram as a framework. An excess of
compact (linear size $\leq 10$~kpc) sources could be explained by low-power
jets being decelerated within the host galaxy, as shown by recent numerical
simulations. Finally, I discuss the possible tracks that radio sources may
follow within this diagram, and the physical processes that can explain the
different tracks.
