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In this study, we explore the design of a subthreshold processor for use in ultra-low-energy sensor systems. We describe an 8-bit subthreshold processor that has been designed with energy efficiency as the primary constraint. The processor, which is functional below Vdd=200 mV, consumes only 3.5 pJ/inst at Vdd=350 mV and, under a reverse body bias, draws only 11 nW at Vdd=160 mV. Process and temperature variations in subthreshold circuits can cause dramatic fluctuations in performance and energy consumption and can lead to robustness problems. We investigate the use of body biasing to adapt to process and temperature variations. Test-chip measurements show that body biasing is particularly effective in subthreshold circuits and can eliminate performance variations with minimal energy penalties. Reduced performance is also problematic at low voltages, so we investigate global and local techniques for improving performance while maintaining energy efficiency.