The neutron diagnostics of large tokamaks like JET are essential to estimate the fusion power and to determine some various parameters. The neutron activation method supported by neutron transport calculations is very useful especially for the evaluation of the total neutron yield from a single plasma discharge. The paper presents some results of the activation experiments and calculations carried out for the JET plasmas, from selection of the activation materials to their irradiations in the neutron field of JET discharges. Neutron transport calculations leading to activation coefficients for new materials have been performed. The results of the calculations have been used to design new composite foils to obtain information also about the neutron spectrum and not only the total yield. Measurements using these new activation materials have been performed during last JET experimental campaigns. The obtained experimental results are compared with the theoretical calculations.