The nascent field of spintronics has great potential to enable new types of information processing and storage devices and supplement conventional semiconductor electronics. An overview of nanoscale thermal phenomena in a tunnel junctions is provided, which is one of the key building blocks of spintronic devices. Experiments showed that the thermal resistance of nanoscale tunnel barriers increases linearly with thickness, which is consistent with the theory of energy transport in highly disordered materials. Heat conduction across a tunnel junction is impeded by significant additional resistance at interfaces between the barrier layer and electrodes due to mismatch in atomic vibrational properties and nonequilibrium between electrons and phonons. The quantum-mechanical tunneling probability depends strongly on electron energy, which leads to asymmetry in heat-generation rate along the two opposing electrodes of a tunnel junction.