Neutron flux and energy spectrum measurements are conducted for all major fusion materials irradiation facilities, including fission reactors and accelerators. Dosimetry characterization experiments and integral cross section measurements have been performed. Multiple activation and helium production measurements are performed routinely to provide materials experimenters with neutron exposure parameters including fluence, spectrum, displacements, gas production, and transmutation with typical accuracies of 10–15%. Such data are crucial to the fusion materials program in order to correlate materials property changes between irradiations and facilities and to confidently predict the performance of materials in fusion reactors.