Primordial nucleosynthesis is one of the three observational evidences for the Big Bang model. Even though they span a range of nine orders of magnitude, there is indeed a good overall agreement between primordial abundances of ^4He, D, ^3He and 7Li either deduced from observation or primordial nucleosynthesis. This comparison was used for the determination of the baryonic density of the Universe. For this purpose, it is now superseded by the analysis of the CMB anisotropies. Big Bang nucleosynthesis remains, nevertheless, a valuable tool to probe the physics of the early Universe.