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 He4, D, He3 and He7 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 For this purpose, it is now superseded by the analysis of the Cosmic Microwave Background radiation anisotropies. Big-Bang nucleosynthesis remains, nevertheless, a valuable tool to probe the physics of the early Universe.