The status of bulk explosive detection techniques based on nuclear interrogation methods is reviewed. The desirable characteristics of an operational system capable of meeting the requirements for civil aviation security are compared with what can be expected from nuclear-based techniques. The comparison is limited to those techniques that utilize penetrating neutron and photon probes on the target elements of explosives most likely to be encountered in the airport scenario. The physical properties of a relevant group of explosives are surveyed for unique characteristics that could provide detectable signatures to nuclear-based techniques. A survey of the accessible reactions and their relative detection sensitivities are tabulated for a selection of practical interrogating probes. Some results obtained with systems currently under development are reported.