The surge in energy requirements has driven the pursuit of sustainable energy sources and advanced methods to optimize energy efficiency. One promising solution is the use of thermoelectric materials to convert waste heat into electrical power, which also aids in mitigating environmental challenges. The performance of thermoelectric devices is heavily influenced by the materials used and their key properties, including the Seebeck coefficient, electrical and thermal conductivity, and thermal stability. Achieving optimal thermoelectric performance requires materials that balance excellent electronic transport with favorable thermal transport properties. Nanotechnology has been instrumental in driving the notable progress made in thermoelectrics over the last few decades. Nanostructuring is particularly important, as it enables the precise tuning of properties needed for exceptional thermoelectric performance. The grain boundaries in nanostructured materials efficiently scatter mid- and long-wavelength phonons, thus reducing thermal conductivity. Nanocrystalline domains also enhance the Seebeck coefficient by modifying the density of states and facilitating type-and energy-dependent charge carrier scattering. The full potential of these advantages can be realized through the design of nanocomposites, with careful control over both structural and chemical parameters at multiple length scales. This chapter explores the strategic design of nanomaterials and composites, emphasizing the enhancement of the thermoelectric figure of merit through the synergistic tuning of the Seebeck coefficient, conductivity, and thermal stability. It discusses key thermoelectric materials, including metal oxides, metal chalcogenides, metal sulfides, metal selenides, MXenes, graphene, polymers, and clathrates. In conclusion, the chapter highlights the applications of thermoelectric materials, discusses existing limitations, and outlines future prospects.