These include high atomic weight (which led to PbTe and Bi2Te3), loosely bound “rattling” atoms (e.g., clathrates), low dimensionality (e.g., nanorods and quantum dots), and phonon localization (e.g., superlattices and alloys). Sincehundredsofmaterialshavebeensystematicallyscreened,tried,and tested, new ideas are required to move this field forward.