This chapter describes the methods used to measure spent nuclear fuel. The first section describes the properties of typical reactor fuel: geometry, burnup, initial enrichment, and cooling time. Neutron multiplication in the assembly is discussed. The measurement methods are divided into gamma-based methods, neutron-based methods, and other methods. A particularly important gamma-based method is passive gamma emission tomography. The neutron-based methods range from the relatively simple passive neutron fork, passive neutron albedo reactivity and differential dieaway to the complex active methods. The other methods are Cerenkov viewing devices, curium/plutonium ratio and calorimetry.