This thesis addresses some of the design concerns facing the receivers for such sensing systems, with a particular focus on improving the sensitivity, reliability, and robustness of ultra-low power receivers. Two prototype radios have been implemented exploring different applications. The first focuses on a general purpose low power receiver for an Active RFID tag, which provides a highly integrated wireless sensing platform including a full transceiver along with power conditioning and a DC-DC converter to interface with an energy harvester. This receiver consumes 48uW of analog power, along with 61uW of digital power for demodulation and synchronization and achieves a sensitivity of -66dBm for 100kbps of data while being powered by the on-chip power supply. The second implements a wakeup radio, which operates along with a more powerful, full-featured radio such as a WiFi or Bluetooth and listens to the channel to determine if signals are present. This design focuses on minimizing the active power consumption while improving the sensitivity, and achieves a -90dBm sensitivity for a 10kbps OOK wakeup signal while consuming only 37.5uW of power.