In this chapter, SiGe nanocomposites are investigated for various parameters, such as thermal conductivity, electrical conductivity, and Seebeck coefficient, which determine their applications in thermoelectrics. Grain boundaries in nanocomposites can scatter phonons, when their mean free path is longer than the grain size. Mean free path of electrons is usually shorter than the grain size of nanocomposites, so that the electrical conductivities of nanocomposites are not expected to change significantly. However, the results show that, at the nanoscale, the properties related to electron transport are affected. Based on the calculations of the electronic and thermal properties in the literature, studies show that an enhancement in ZT for n-type and p-type SiGe alloys is mostly due to the reduction in the thermal conductivity. Such a reduction is due to both the alloying effect and increased phonon interface scattering at the grain boundaries.