Deuterium-tritium (D-T) neutron-induced radioactivity constitutes one of the foremost issues infusion reactor design. Designers have been using radioactivity codes and associated nuclear data libraries for nucleonic designs of fusion reactors. However, in the past, there was hardly any experimental validation of these codes/libraries. An elaborate, experimental program was initiated in 1988 under a U.S. Department of Energy/Japan Atomic Energy Research Institute collaborative program to validate the radioactivity codes/libraries. Measurements of decay gamma spectra from irradiated, high-purity samples of Al, Si, Ti, V, Cr, Mn-Cu alloy, Fe, Co, Ni, Cu, SS316/AISI316, Zn, Zr, Nb, Mo, In, Sn, Ta, W, and Pb, among others, have been carried out under D-T neutron fluences ranging from 1.6 × 1010 to 6.1 × 1013 n/cm2 and cooling times ranging from ∼10 min to ∼3 weeks. As many as 14 neutron energy spectra were covered for a number of materials. The analyses of the isotopic activities of the irradiated materials using the activation cross-section libraries of four leading radioactivity codes, i.e., ACT4/THIDA-2, REAC-3, DKR-ICF, and RACC, have shown large discrepancies among the calculations on one hand and between the calculations and the measurements, on the other. Vanadium, Co, Ni, Zn, Zr, Mo, In, Sn, and W each count the largest number of discrepant isotopic activities. It is strongly recommended to continue additional radioactivity experiments under additional neutron energy spectra and large neutron fluence on one hand and to improve activation cross sections related to the problematic isotopic activities on the other. A unique activation cross-section library and associated radioactivity code are also recommended for the best results. In addition to providing detailed results of the status of predictability of individual isotopic activities using the ACT4, REAC-3, DKR-ICF, and RACC activation cross-section libraries, safety factors cum quality factors characterizing each library are presented and discussed. The related issues of confidence level and associated uncertainty are also highlighted. These considerations are of direct practical importance to reactor designers.