  Giant radio sources form the linear size extreme of the extra-galactic radio
source population. Using the WENSS survey, we have selected a complete sample
of these sources. We have investigated the properties of their radio
structures. We find, among other things, that these sources are old (50-100
Myr) and have higher advance velocities than smaller sources of similar radio
power. We find pressure gradients in their radio lobes, suggesting that the
lobes are still overpressured with respect to the environment. Further, we find
no evidence for a cosmological evolution of the radio lobe pressures with
increasing redshift, at least up to $z\sim 0.4$, other than that caused by
selection effects. We argue that a much fainter sample of giant sources than
currently available is needed to constrain the pressure in their environments,
the IGM. Another extremely important discovery is that of a population of radio
sources with a so-called `double-double' structure, i.e. that of a small
two-sided radio source embedded inside a much larger two-sided structure. We
argue that these sources result from an interrupted central jet-forming
activity. As such, they are the most convincing examples of radio sources with
a history of interrupted activity, yet. Since the inner lobes advance within
the outer lobes, high resolution low frequency ($\sim 200$ MHz) polarization
studies may reveal the constituents of radio lobes and cocoons. We thus argue
for a SKA design that can provide low-frequency images at arcsec resolution,
but which is also sensitive to structures as large as a few tens of arcminute
on the sky.
