DARPA’s Defense Sciences Office is requesting information on the current state of the art and technical limitations of solid-state direct thermal-to-electric energy conversion technologies, defined as systems converting a temperature differential directly into electricity without mechanical moving parts. The RFI seeks to identify the best possible solid-state heat engine performance across a wide operating temperature range, the factors limiting that performance, and the most attractive use cases if highly efficient solid-state heat engines could be made today. The resulting survey of limits, challenges, and use cases will identify research and development opportunities to inform potential future programs in this area.