In order to achieve a detailed microscopic understanding of heat transport in thermoelectrics, a detailed knowledge of the phonon mean-free-paths is needed. We use neutron and x-ray scattering techniques to probe the phonon dispersions and scattering rates across the entire Brillouin zone. Neutron spectrometers can efficiently map phonon excitations throughout reciprocal space, providing critical information about the many possible phonon scattering mechanisms, including anharmonicity, electron-phonon coupling, and scattering by defects or nanostructures. X-rays enable complementary phonon measurements, and also reveal details of the nanostructure through the diffuse scattering. These neutron and x-ray measurements are complemented with bulk transport measurements and first-principles simulations to establish an atomistic understanding of phonon scattering mechanisms, and their effect on thermal conductivity. I will present investigations of phonons in several important thermoelectric materials, including PbTe, SnTe, and AgSbTe 2 , establishing a microscopic understanding of their thermal conductivity [1,2,3]. These insights suggest avenues to improve the performance of thermoelectric materials.
 J. Ma * , O. Delaire * , A. F. May, C. E. Carlton, M. A. McGuire, L. H. VanBebber, D. L. Abernathy, G. Ehlers, Tao Hong, A. Huq, Wei Tian, V. M. Keppens, Y. Shao-Horn, and B. C. Sales, Nature Nanotechnology 8 , 445 (2013).
 C.W. Li, O. Hellman, J. Ma, A.F. May, H.B. Cao, X. Chen, A.D. Christianson, G. Ehlers, D.J. Singh, B.C. Sales, and O. Delaire, Physical Review Letters (2014).
 O. Delaire, J. Ma, K. Marty, A. F. May, M. A. McGuire, M.-H. Du, D. J. Singh, A. Podlesnyak, G. Ehlers, M. Lumsden, B. C. Sales, Nature Materials 10 , 614 (2011).
Funding from the US DOE, Office of Basic Energy Sciences, Materials Science and Engineering Division, and as part of the S3TEC EFRC. Modeling of neutron data in CAMM was funded by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.