Abstract : Some neutrons are emitted from fission products seconds to minutes after fission occurs. The knowledge of these delayed neutrons is essential in the field of nuclear energy. But the probabilities to emit such delayed neutrons (Pn) are not always well known. A summary of different databases and compilations of Pn values is presented to show these discrepancies and uncertainties. The usual methods used to determine these nuclear data are then reviewed with an emphasis on biases and systematic errors to be avoided. To measure precise Pn values, a new neutron LOng-counter with ENergy Independant Efficiency (LOENIE) has been built for the LOHENGRIN separator facility installed at Institut Laue Langevin (FRANCE). Its characteristics and first results obtained are presented.