Despite the unreproducibility, doubt, and controversy involved in the question of the “cold fusion” of deuterium, enough good data have been published to clearly indicate the reality of some sort of nuclear fusion. Yamaguchi and Nishioka reported a thrice-repeated event in which large amounts of heat and definite bursts of neutrons evolved simultaneously with considerable out-gassing of absorbed deuterium. These results are consistent with nuclear fusion and not with a chemical reaction. A detailed mechanism is proposed that is consistent with these events and that also generally explains many of the scattered indications of cold fusion that have been reported. There must be an adventitiously large enough presence of tritium to initiate the nuclear reaction. The results of previously successful experiments cannot now be reproduced because currently available D2O (and D2) is so low in adventitious tritium as to preclude initiation of the nuclear reaction.