  Radio-loud compact radio sources (CRSs) are characterised by morphological
compactness of the jet structure centred on the active nucleus of the galaxy.
Most of the local elliptical galaxies are found to host a CRS with nuclear
luminosities lower than those of typical quasars, $\lesssim$10$^{42}\, {\rm
erg\, s}^{-1}$. Recently, low-luminosity CRSs with a LINER-like optical
spectrum have been named Fanaroff-Riley (FR) type 0 to highlight their lack of
substantially extended radio emission at kpc scales, in contrast with the other
Fanaroff-Riley classes, full-fledged FRIs and FRII radio galaxies. FR0s are the
most abundant class of radio galaxies in the local Universe, and characterised
by a higher core dominance, poorer Mpc-scale environment and smaller (sub-kpc
scale, if resolved) jets than FRIs. However, FR0s share similar host and
nuclear properties with FRIs. A different accretion-ejection paradigm from that
in place in FRIs is invoked to account for the parsec-scale FR0 jets. This
review revises the state-of-the-art knowledge about FR0s, their nature, and
which open issues the next generation of radio telescopes can solve in this
context.
