Neutrinos can play an important role in the evolution of the universe, modifying some of the cosmological observables. In this contribution we summarize the main aspects of cosmological relic neutrinos, and we describe how the precision of present cosmological data can be used to learn about neutrino properties, in particular their mass, providing complementary information to beta decay and neutrinoless double-beta decay experiments. We show how the analysis of current cosmological observations, such as the anisotropies of the cosmic microwave background or the distribution of large-scale structure, provides an upper bound on the sum of neutrino masses of order 1？eV or less, with very good perspectives from future cosmological measurements which are expected to be sensitive to neutrino masses well into the sub-eV range. 1. Introduction The subject of this contribution is the role of neutrinos in cosmology, one of the best examples of the very close ties that have developed between nuclear physics, particle physics, astrophysics, and cosmology. Here we focus on the most interesting aspects related to the case of massive (and light) relic neutrinos, but many others that were left out can be found in [1, 2]. We begin with a description of the properties and evolution of the background of relic neutrinos that fills the universe. Then we review the possible effects of neutrino oscillations on cosmology. The topic of neutrinos and Big Bang Nucleosynthesis is reviewed in a different contribution to this special issue [3]. The largest part of this contribution is devoted to the impact of massive neutrinos on cosmological observables that can be used to extract bounds on neutrino masses from present data. Finally we discuss the sensitivities on neutrino masses from future cosmological experiments. Note that massive neutrinos could also play a role in the generation of the baryon asymmetry of the universe from a previously created lepton asymmetry. In these leptogenesis scenarios, one can also obtain quite restrictive bounds on light neutrino masses, which are, however, strongly model dependent. We do not discuss this subject here, it is covered in other contribution to this special issue [4]. For further details, the reader is referred to recent reviews on neutrino cosmology such as [5–7] and in particular [8]. A more general review on the connection between particle physics and cosmology can be found in [9]. 2. The Cosmic Neutrino Background The existence of a relic sea of neutrinos is a generic feature of the standard hot big bang model, in number only
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