Thermoelectrics is an emerging technology for effective recovery of waste heat from power plants, factories, motor vehicles, computers or even human bodies and its conversion into useful electricity without moving parts, noise or the release of harmful chemicals. Therefore, such converters exhibit a high potential to contribute to the global efforts to reduce CO2 emissions from fossil fuels and towards a cleaner and sustainable environment. Currently available converters contain scarce, expensive and environmentally unfriendly elements. They can only work in a limited temperature range, resulting in limited efficiencies of only up to several percent. In this talk the potential of both n– and p– type novel and highly efficient materials including the TiNiSn- based half-Heusler, HH, compounds, will be described via understanding the role and controlling of meso-scale defects, such as grain-boundaries, separated phases, dislocations/twinning and chemical impurities on the thermoelectric properties. Such materials, containing available, cost-effective and environmentally friendly constituents, are capable of operation up to 700oC, and therefore expected to enhance the Carnot efficiency with all of the other associated advantages. The efforts of maximizing the potential of such materials by both electronic and phonon phonon effects will be described in detail in the talk and the routes approaching practical energy conversion devices will be addressed.