Solid-state thermoelectric devices have the ability to directly convert heat into electricity for power generation. A good TE material should possess a high electrical conductivity, a high Seebeck coefficient, and a low thermal conductivity. This requirement is hard to be satisfied within the same material. To address this issue, the nanostructuring approach has been widely used to reduce the lattice part of the thermal conductivity (kL) but still maintain the bulk electrical properties. High TE performance has thus been achieved in various nanostructured materials, such as nanoporous thin films and different nanostructured bulk materials. To better understand the observed kL reduction by nanostructures, two major types of nanostructured materials are studies here, including nanoporous thin films and nanograined bulk materials. In the latter case, a super-flexible thin film was hot pressed onto a wafer to represent a grain boundary.