A method for the nondestructive assay of spent light water reactor (LWR) fuel assemblies based on combined active and passive neutron counting is presented. The method allows the determination of burn-up, total fissile content, original enrichment of the spent fuel, and type of fuel [uranium or mixed oxide (MOX)]. The method, which was originally developed for criticality control in the front end of a reprocessing plant, can be used for plant safety assurance in nuclear installations and fuel storage facilities and for safeguards purposes. Measurements on spent uranium and MOX LWR fuel assemblies were undertaken in storage ponds at reprocessing plants and power stations. Results and experiences of the demonstration program are presented. Without prior knowledge of any fuel assembly data, the burnup of uranium fuel assemblies can be determined with an uncertainty of ±1200 MWd/tonne U and the initial enrichment of uranium fuel assemblies with an accuracy of ±5%. Using these data and accuracies, the total plutonium content can be determined from isotopic correlations with an accuracy of better than ±0.3 kg/tonne U for pressurized water reactor and ±0.5 kg/tonne U for boiling water reactor fuel assemblies.