We present the architecture of an 802.15.4 receiver that, for the rsttime, operates at a few hundred microwatts, enabling new batteryfreeapplications. To reach the required micro-power consumption,the architecture diverges from that of commodity receivers in twoimportant ways. First, it ooads the power-hungry local oscillatorto an external device, much like backscatter transmitters do. Second,we avoid the energy cost of demodulating a phase-modulatedsignal by treating 802.15.4 as a frequency-modulated one, whichallows us to receive with a simple passive detector and an energyecientthresholding circuit. We describe a prototype that canreceive 802.15.4 frames with a power consumption of 361 µW. Ourreceiver prototype achieves sucient communication range to integratewith deployed wireless sensor networks (WSNs). We illustratethis integration by pairing the prototype with an 802.15.4 backscattertransmitter and integrating it with unmodied 802.15.4 sensornodes running the TSCH and Glossy protocols.