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., superlatticesandalloys). Since hundreds of materials hav ebeen system atically screened,tried,and tested,new ideas are required to move this field forward.