  New measurements of the cosmic microwave background (CMB) by the Planck
mission have greatly increased our knowledge about the Universe. Dark
radiation, a weakly interacting component of radiation, is one of the important
ingredients in our cosmological model which is testable by Planck and other
observational probes. At the moment the possible existence of dark radiation is
an unsolved question. For instance, the discrepancy between the value of the
Hubble constant, H_0, inferred from the Planck data and local measurements of
H_0 can to some extent be alleviated by enlarging the minimal LambdaCDM model
to include additional relativistic degrees of freedom. From a fundamental
physics point of view dark radiation is no less interesting. Indeed, it could
well be one of the most accessible windows to physics beyond the standard
model. An example of this is that sterile neutrinos, hinted at in terrestrial
oscillation experiments, might also be a source of dark radiation, and
cosmological observations can therefore be used to test specific particle
physics models. Here we review the most recent cosmological results including a
complete investigation of the dark radiation sector in order to provide an
overview of models that are still compatible with new cosmological
observations. Furthermore we update the cosmological constraints on neutrino
physics and dark radiation properties focussing on tensions between data sets
and degeneracies among parameters that can degrade our information or mimic the
existence of extra species.
