Optical 3D imagers for gesture recognition suffer from large size and high power consumption. Their performance depends on ambient illumination and they generally cannot operate in sunlight. These factors have prevented widespread adoption of gesture interfaces in energy- and volume-limited environments such as tablets and smartphones. Wearable mobile devices, too small to incorporate a touchscreen more than a few fingers wide, would benefit from a small, low-power gestural interface. Gesture recognition using sound is an attractive alternative to overcome these difficulties due to the potential for chip-scale size, low power consumption, and ambient light insensitivity. Using pulse-echo time-of-flight, MEMS ultrasonic rangers work over distances of up to a meter and achieve sub-mm ranging accuracy [1,2]. Using a 2-dimensional array of transducers, objects can be localized in 3 dimensions. This paper presents an ultrasonic 3D gesture-recognition system that uses a custom transducer chip and an ASIC to sense the location of targets such as hands. The system block diagram is shown in Fig. 12.1.1. Targets are localized using pulse-echo time-of-flight methods. Each of the 10 transceiver channels interfaces with a MEMS transducer, and each includes a transmitter and a readout circuit. Echoes from off-axis targets arrive with different phase shifts for each element in the array. The off-chip digital beamformer realigns the signal phase to maximize the SNR and determine target location.