Cosmic microwave background as a probe of dark relics

I argue how cosmology (and the CMB in particular) is sensitive to even extremely suppressed interaction rates of (meta)stable species present in the cosmic soup. The case of dark relics decaying either in a dark sector or via some tiny visible branching ratio into visible form are reviewed. Applications to evaporating and merging primordial black holes are also covered. Finally, I conclude with some considerations on the CMB sensitivity to dark radiation.