Overhead associated to the link establishment and operation is a major source of inefficiency in practical millimeter wave (mmWave) systems. Sources of significant overhead including raining transmit and receive beams during the processes of beam training, channel estimation and feedback and also channel tracking. We propose to investigate the potential for communicating more efficiently based on side information obtained from sensors or from out-of-band measurements available in other communication systems operating in parallel, like in multiband base stations. This talk explains the key idea of out-of-band communication and highlights some recent work in exploiting this form of side information to assist compressed beam training.