The objective of this proposal is to develop high efficiency thermoelectric composites. The team proposes a new hierarchical multiscale strategy to develop high efficiency thermoelectric composites that build upon atomistic-nano-continuum computation guided material design; interfacial modification techniques; bulk functional gradient approaches; and nano-to-continuum characterization methodologies. These methodologies will be applied to solve critical problems of designing and developing high efficiency thermoelectric composites. The main tasks include (a) determining the optimized atomistic composition, molecular surface modification, and macroscopic morphology with first-principles, perturbation theory, and continuum modeling; (b) synthesizing bulk thermoelectric composites containing nano-scale grain with surface modifications and macroscopic functional gradients; and (c) characterizing electron and phonon transport from the molecular to the macro-scale. These tasks extend existing modeling and experimental capabilities, provide new understanding of interfacial and functional gradient electron and phonon scattering mechanisms, and directly interface with industrial development of thermoelectric waste heat recovery technology for improved fuel economy.