This dissertation investigates the use of the harmonic radar technique for passive wireless sensing applications. Issues of DC power consumption, high RF activation power, large node size, and short communication range associated with the existing passive wireless sensing technologies are addressed by the development of novel, completely passive, high efficiency, compact 3-D harmonic sensor nodes. The node transceiver employs a passive frequency doubler to return the second harmonic of the interrogation signal, and electrically-small 3-D antennas to achieve the compactness and high efficiency. The developed nodes fit inside a sphere with a diameter < 3 cm and achieve communication range > 60 m using a 43 dBm EIRP interrogator. Effective modulation is demonstrated experimentally using low cost commercial vibration sensors. To address major challenges associated with long-range, embedded, passive wireless sensing including sensor node identification and remote channel calibration, a 3-D dual-channel transceiver is developed. To the best of the author's knowledge, the presented dual-channel transceiver is the first completely passive design with built-in passive remote channel calibration and identification capabilities, and the presented nodes have the best overall performance among previously published designs, in terms of conversion efficiency, communication range, and occupied volume. To reduce the cost and weight and improve the manufacturing process of the proposed nodes, the 3-D digital additive manufacturing and conformal direct printing technologies are employed. The harmonic interrogator antenna design is also an underlying focus of this work. Different interrogator antenna candidates are developed based on different design approaches. The first approach is based on the use of dual-channel antenna array, where one channel is used for transmission and the second channel is used for reception. Two dual-channel harmonic interrogator antennas that consist of 4-element circular patch antenna arrays and 2-element quasi-Yagi dipole antenna arrays are implemented. The second approach employs mechanically reconfigurable antennas to reduce the size and maintain persistent radiation properties over wide frequency bandwidth. Two mechanical reconfiguration methods are developed; the first method is based on the use of Hoberman's planar foldable linkage to vary the operating frequency of planar circular patch antennas and the second mechanical reconfiguration method is based on the use of a rack and pinion mechanism to reconfigure dual-band slot antennas. The third approach employs a single channel multi-octave Vivaldi antenna to provide the capability to interrogate a large number of harmonic tags that are widely spaced in frequency. To improve the antenna radiation performance over a broad frequency range, a new method based on the introduction of a parasitic elliptical patch in the flare aperture is proposed. This method enables gain and bandwidth improvement compared to what has been reported for Vivaldi antennas with a compact size. To provide the interrogator the capability to steer the radiation beam for locating and tracking sensor nodes, a topology to develop a miniature, non-dispersive switchable 4-bit phase shifter is proposed on the basis of composite right/left handed transmission line unit cells.