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Dark radiation, parameterized in terms of $N_{\rm eff}$, has been consideredmany times in the literature as a possible remedy in alleviating the Hubbleconstant ($H_0$) tension. We review here the effect of such an extra darkradiation component in the different cosmological observables, focusing mostlyon $H_0$. While a larger value of $N_{\rm eff}$ automatically implies a largervalue of the Hubble constant, and one would naively expect that such a simplescenario provides a decent solution, more elaborated models are required. Lightsterile neutrinos or neutrino asymmetries are among the first-order correctionsto the most economical (tree-level) massless dark radiation scenario. However,they are not fully satisfactory in solving the $H_0$ issue. We devote herespecial attention to second-order corrections: some interacting scenarios, suchas those with new dark radiation degrees of freedom that exhibit a non-freestreaming nature are highly satisfactory alternative cosmologies where to solvethe Hubble constant tension. Models with self-interacting sterile neutrinosand/or majorons, both well-motivated beyond the Standard Model particles, willbe discussed along our assessment.

On the dark radiation role in the Hubble constant tension