Nanostructured materials and thermoelectrics have been the subject of significant research in recent years. Bulk nanostructured thermoelectric materials are attractive as they may be produced in sufficient quantity to be of potential use in large-scale applications, whereas superlattice or quantum dot structures are currently able to be produced only as thin films, limiting their applicability. The formation of a bulk nanocomposite is an effective way of lowering thermal conductivity for the enhancement of thermoelectric performance. This chapter discusses the principles of thermoelectric energy conversion and introduces the concept of bulk nanostructured materials. A structural phase transition near 420 K in Ag₂Te raises the interface potential effect, which implies its dispersion in the Bi₂Te₃-, Sb₂Te₃- and PbTe-based matrices. Composites synthesised by controlling the shape of Ag₂Te particles with commercial (C), nanowire (NW) and nanopowder (NP) materials, and changing the dispersion concentration in the Bi₂Te₃ and Sb₂Te₃ matrices, show a well-ordered thermoelectric behaviour. By considering the interface potential and lattice mismatch, a charge-selective Anderson localisation while preserving the extended electronic state is introduced. From this approach, an exceptionally high thermoelectric figure of merit of ZT > 2.0 in n-type PbTe materials was successfully achieved, suggesting that this may be a versatile approach for power generation from waste heat.