This SBIR Phase I proposal responds to topic S3.03 of the 2010 NASA SBIR solicitation, for Power Generation and Conversion. Thermoelectric devices offer a simple and reliable means to convert radioisotope thermal energy into useable electrical power. Previously, thermoelectric devices based on bulk semiconductor materials have been limited by low conversion efficiencies, with Figure of Merit (ZT) values around 1.0 or less. Increasing ZT above 1.0 has thus far proved difficult, due to the fundamental limitation of identifying semiconductor materials with both a high electrical conductivity and low thermal conductivity. This SBIR project will develop high efficiency thermoelectric devices based on nanowires. The use of nanotechnology provides a means to circumvent previous limitations, and achieve combinations of properties not possible with bulk materials. Phase I will demonstrate technical feasibility by producing high efficiency thermoelectric devices based on nanowires. In Phase II, we will build and demonstrate prototype high efficiency thermoelectric devices. Phase II will also develop low cost manufacturing technology for the nanowire based thermoelectric devices, and demonstrate a technology readiness level of TRL 6.