A large body of experimental observations has evolved with particular reference to deuterated palladium, a mechanism of fusion unique to condensed matter. The mechanism brings to focus the relevance of the electronic structure of the host lattice, indicating the features that are desired. Direct interaction of electronegative elements such as oxygen (as happens in electrolysis experiments) creates, through modification of the electronic structure, situations under which heavy electrons are manifested. In cases where an oxide interface is present, an analogous situation is created at the onset of an insulator-metal transition caused by the induced migration of deuterons through the layer. Screened by the heavy fermions, deuterons in such situations undergo transition to a more stable quasi-molecular state, (D+D+)2e−, with substantially reduced nuclear separation. Through quantum mechanical tunneling, fusion takes place in such a cluster with a yield of 10−1.5 s−1, a value consistent with observed excess heat production and near-surface occurrence of the phenomenon.