In this work, we report the first study of beamforming on mobile devices. We first show that beamforming is already feasible on mobile devices in terms of form factor, power efficiency and device mobility. We then investigate the optimal way of using beamforming in terms of power efficiency, by allowing a dynamic number of active antennas. We propose a simple yet effective solution, BeamAdapt, which allows each mobile client in a network to iteratively identify the optimal number of active antennas with fast convergence and close-to-optimal performance. Finally we report a WARP-based prototype of BeamAdapt and experimentally demonstrate its effectiveness in realistic environments. We also complement the prototype-based experiments with Qualnet-based simulation of a large-scale network. Our results show that BeamAdapt with four antennas can reduce the power consumption of mobile clients by more than half compared to omni directional transmission, while maintaining a required network throughput.