Nanocomposite thermoelectric materials have attracted much attention recently due to experimental demonstrations of improved thermoelectric properties over those of the corresponding bulk material. While enhanced thermoelectric properties have been demonstrated in other nanostructured materials, nanocomposites are unique because they can be fabricated quickly, cheaply, and in a form that is compatible with existing thermoelectric device configurations. Because of this practicality, there is significant interest in using these materials for thermoelectric devices, and commercialization efforts are ongoing. However, creating materials with further improved thermoelectric properties will require a better understanding of carrier transport in these complex materials. In this paper, we examine important features of transport in bulk nano structured materials using the Boltzmann equation. We account for the strong grain boundary scattering mechanism in nano composite s by introducing grain boundary scattering models which yield electron and phonon grain boundary scattering rates. We then apply the model to a Si_xGe_1−x nanocomposite and show that the calculated results are in good agreement with recently reported measurements. We also use the model's predictions to help determine which strategies will be most effective in further improving the figure of merit of these materials.