  We have derived new bounds on the relativistic energy density in the Universe
from cosmic microwave background (CMB), large scale structure (LSS), and type
Ia supernova (SNI-a) observations. In terms of the effective number of neutrino
species a bound of N_\nu = 4.2^{+1.2}_{-1.7} is derived at 95% confidence. This
bound is significantly stronger than previous determinations, mainly due to
inclusion of new CMB and SNI-a observations. The absence of a cosmological
neutrino background (N_\nu = 0) is now excluded at 5.4 \sigma. The value of
N_\nu is compatible with the value derived from big bang nucleosynthesis
considerations, marking one of the most remarkable successes of the standard
cosmological model. In terms of the cosmological helium abundance, the CMB,
LSS, and SNI-a observations predict a value of 0.240 < Y < 0.281.
