This proposal describes the development of passive wireless surface acoustic wave (SAW) based liquid level sensors for NASA application to cryogenic liquid level sensing. Orthogonal Frequency Coded (OFC) SAW devices have been demonstrated as passive wireless temperature sensors in NASA Contract NNK04OA28C, and are being further developed under NNK05OB31C. The proposed liquid level sensors will use damping of the acoustic wave caused by mass loading of the liquid to produce fast, reversible liquid level sensors. The proposed research will apply the results of ongoing work on OFC sensors and will evaluate their operation as liquid level sensors in selected liquids of interest at cryogenic temperatures. Issues to be investigated include the stability of various SAW substrates when exposed to various (extreme and gradual) temperature changes, the effects on device integrity and performance of thermal cycling to cryogenic temperatures, and the effects of device exposure to various liquids and the reversibility thereof. The result of the proposed research will be an understanding of potential failure mechanisms in SAW sensors used at cryogenic temperatures, knowledge of the required operating parameters to ensure device reliability under likely operating conditions, and demonstration of the use of OFC SAW sensors for liquid level detection.