Radioisotope Thermoelectric Generators (RTG) have been used by NASA to reliably power spacecraft for deep space exploration for over 40 years. Current state of the practice systems are limited to device-level efficiencies of 7.5% or less and system level specific powers of 2.8 to 5.1 W/Kg. NASA’s Radioisotope Power Systems Thermoelectric Technology Development Program (TTDP) is pursuing development of more efficient thermoelectric technologies that can increase performance by a factor of 2 to 4x over these state of the practice systems1. NASA’s TTDP is developing high-efficiency segmented couples/modules with the following design goals: A) system conversion efficiency ≥ 11% (≥ 60% improvement over MMRTG at BOL) and b) ≥ 6-8.5 We/kg specific power (2-3 x improvement over MMRTG), for a temperature gradient T = 800 K (TH=1273 K and TC = 473 K). We will be discussing the state of development of the aforementioned couples and the tools that are used to guide this development. First-principle calculations and Finite Element Thermomechanical analysis are used to guide materials selection and device architecture, while extended device testing is utilized to establish couple reliability and stability.