BeamBand is a wrist-worn system that uses ultrasonic beamforming for hand gesture sensing. Using an array of small transducers, arranged on the wrist, we can ensem-ble acoustic wavefronts to project acoustic energy at spec-ified angles and focal lengths. This allows us to interro-gate the surface geometry of the hand with inaudible sound in a raster-scan-like manner, from multiple view-points. We use the resulting, characteristic reflections to recognize hand pose at 8 FPS. In our user study, we found that BeamBand supports a six-class hand gesture set at 94.6% accuracy. Even across sessions, when the sensor is removed and reworn later, accuracy remains high: 89.4%. We describe our software and hardware, and future ave-nues for integration into devices such as smartwatches and VR controllers.