Nanostructured materials can be used to improve the performance of thermoelectric materials. The theories required to understand how nanostructures affect the electric conductivities, thermal conductivities and Seebeck coefficients will be reviewed before examples will be provided of the enhanced performance that can be achieved through low-dimensional structures. Some of the best thermoelectric material performance results from the literature will be presented and compared to the best bulk material results. A review will also be presented of microfabricated thermoelectric generators with explanations of the key output parameters and how these parameters can be optimised for applications. Microfabricated thermoelectric generators require less material than conventional pick-and-place systems allowing low-cost and higher power density thermal energy harvesting systems.