Book excerpt: Piezoelectric and thermoelectric materials represent emerging cutting-edge technological materials for energy harvesting for high-value-added applications. Although these materials have been exhaustively exploited for decades, researchers around the world continue to find technological and scientific innovations that must be disseminated to the engineers of yesterday, today, and tomorrow. Piezoelectric materials, through mechanical stresses applied to them, are capable of generating electricity, while thermoelectric materials are capable of producing electricity thanks to the heat applied to them. Therefore, the direct application of these materials is in energy harvesting, which, together with the reduction of materials, leads them to portable and wearable functional applications. The purpose of this work is to disseminate some of the latest scientific and technological advances by different researchers around the world in the development of devices and applications based on these materials. The book compiles state-of-the-art fundamentals, current uses, as well as emerging applications of piezoelectric and thermoelectric materials. It is a source of inspiration for continued scientific research on the commercial, industrial, and military applications of these materials. Furthermore, it is a valuable and informative resource for undergraduate and graduate students, as well as experts and researchers in the field.