  Compact steep spectrum (CSS) and GHz-peaked spectrum (GPS) radio sources
represent a large fraction of the extragalactic objects in flux density-limited
samples. They are compact, powerful radio sources whose synchrotron peak
frequency ranges between a few hundred MHz to several GHz. CSS and GPS radio
sources are currently interpreted as objects in which the radio emission is in
an early evolutionary stage. In this contribution I review the radio properties
and the physical characteristics of this class of radio sources, and the
interplay between their radio emission and the ambient medium of the host
galaxy.
