The active nondestructive assay techniques for nuclear materials have progressed. The basic physical principles underlining the two main techniques are based on photon interrogation and neutron interrogation. The basic features of the nuclear processes which pertain to the detection and determination of the nuclear materials are the interactions of neutrons and photons with the nuclear materials and with the non-nuclear matrix materials which usually accompany the former. Few ways to partially reduce the effect of the nonnuclear materials on the results of the assay are available. Further theoretical and experimental work is required.