  Neutrinos are the least known particle in the Standard Model of elementary
particle physics. They play a crucial role in cosmology, governing the
universe's evolution and shaping the large-scale structures we observe today.
In this chapter, we review crucial topics in neutrino cosmology, such as the
neutrino decoupling process in the very early universe. We shall also revisit
the current constraints on the number of effective relativistic degrees of
freedom and the departures from its standard expectation of 3. Neutrino masses
represent the very first departure from the Standard Model of elementary
particle physics and may imply the existence of new unexplored mass generation
mechanisms. Cosmology provides the tightest bound on the sum of neutrino
masses, and we shall carefully present the nature of these constraints, both on
the total mass of the neutrinos and on their precise spectrum. The ordering of
the neutrino masses plays a major role in the design of future neutrino mass
searches from laboratory experiments, such as neutrinoless double beta decay
probes. Finally, we shall also present the futuristic perspectives for an
eventual direct detection of cosmic, relic neutrinos.
