 Thermoelectrics, in particular solid state conversion of heat to electricity, is expected to be a key energy harvesting technology to power ubiquitous sensors and wearable devices in the future. A comprehensive review is given on the principles and advances in the development of thermoelectric materials suitable for energy harvesting power generation, ranging from organic and hybrid organic inorganic to inorganic materials. Examples of design and applications are also presented. This article was authored by Johannes Petzekurekis, Klas Thai Brandt, Xavier Crispin, and others.