This thesis presents circuit-level techniques for soft error mitigation, low-power design with performance trade-off, and variation-tolerant low-power design. The proposed techniques are divided into two broad categories. First, error compensation techniques, which are used for soft error mitigation and also for low-power operation of linear and non-linear filters. Second, a framework for variation tolerant low-power operation of wireless devices is presented. This framework analyzes the effects of circuit "tuning knobs" such as voltage, frequency, wordlength precision, etc. on system performance, and power efficiency. Process variations are considered as well, and the best operating tuning knob levels are determined, which results in maximum system wide power savings while keeping the system performance within acceptable limits. Different methods are presented for variation-tolerant and power-efficient wireless communication. Techniques are also proposed for application driven low-power operation of the OFDM baseband receiver.