Nitrogen is one of the most abundant element in our Galaxy with a rich interstellar chemistry, with ammonia resting at the prow of the nitrogen bearing molecule list. Complex molecules carrying the amine functional group have been detected in the interstellar medium, such as the amino acetonitrile, a probable precursor of the glycine amino-acid detected in cometary samples. Nitrogen has only two isotopologues, namely 14N and 15N, and the ratio of these two elements has triggered considerable debates, essentially in the cosmochemistry community. This is because measurements of the 14N/15N isotopic ratio in a wide sample of cosmomaterials and Solar System bodies show large variations, from 50 to 500, and also because there was no clear connection with any of these values with what is measured in precursors of the Solar System, the dark clouds. For at least all these reasons, understanding the formation of nitrogen bearing molecules is certainly an central issue in astrochemistry, cosmochemistry, and astrobiology. In this talk, I will present new measurements of the 14N/15N obtained from HCN and CN observations towards dark clouds. The HCN/HC15N ratio is found in good agreement with ratios measured in comets, whislt the CN/C15N is not. This two results are discussed, in the light of model calculations of the fractionation of nitrogen. A simple scenario, based on the chemistry of nitrogen in dark clouds. is also proposed that explains the large variations of the 14N/15N ratio in the Solar System.