The microstructural defects produced in molybdenum by neutron irradiation in the temperature range 50 °C to 800 °C have been characterized by transmission electron microscopy. At a low irradiation temperature, 50 °C, a large number of small dislocation loops, presumably interstitial, form within a complex dislocation network. At intermediate temperatures, 400–600 °C, small interstitial loops agglomerate into rafts. At high temperatures, 600–800 °C, the small loops which comprise a raft are sufficiently mobile to form large loops which interact with each other to produce a coarse dislocation network. The migration of small loops through the lattice by a combination of prismatic glide and conservative climb accounts for the observed microstructures. Vacancy loops are the only identifiable vacancy defects at temperatures ≥ 500. Between 575–650 °C, both vacancy loops and voids exist. Above 650 °C to at least 800 °C, voids are the exclusive vacancy defect. Information on the effect of impurities and grain boundaries on the configuration and distribution of visible defects is presented.