GPS/CSS radio sources and their relation to other AGN
  We are entering a new era of sensitive, large-area and multi-frequency radio
surveys that will allow us to identify Gigahertz-Peaked Spectrum (GPS) and
Compact Steep Spectrum (CSS) radio sources over a wide range in radio
luminosity and study them within the context of the overall radio-source
populations to which they belong. 'Classical' GPS/CSS objects are extremely
luminous radio sources with a compact double morphology, commonly thought to
represent the earliest stages in the life cycle of powerful radio galaxies
(e.g. O'Dea 1998). It is now becoming easier to identify GPS/CSS candidates
with much lower radio luminosity - particularly in the nearby Universe. These
less powerful objects, with typical 1.4 GHz radio luminosities of $10^{23}$ to
$10^{25}$ W/Hz, include peaked-spectrum radio sources with a core-jet
morphology on parsec scales as well as high-frequency GPS-like peaked
components embedded within lower-frequency extended emission. In the latter
case, the presence of a young GPS component may not be evident from
low-frequency data alone. Many radio galaxies in the local Universe have a
compact (FR-0) morphology, and appear to lack extended radio emission on
kiloparsec scales. The relationship of these FR-0 objects to the classical
GPS/CSS radio sources remains unclear - some of them may represent short-lived
episodes of AGN activity that will not lead to an extended FR-1 or FR-2 radio
galaxy. Future wide-band radio surveys will shed more light on this - such
surveys should ideally be coordinated to cover the full frequency range from
100 MHz to 100 GHz in order to sample all stages of GPS/CSS evolution in an
unbiased way.
