This special issue of the Proceedings of the IEEE focuses on the challenges and opportunities related to future access to the radio spectrum. The importance of this topic is underscored by the tremendous growth in spectrum-consuming technology, driven in part by the global growth in smartphones and tablets. But demand is also driven by an enormous variety of other important applications, such as command and payload signals for unmanned aerial vehicles; vehicular radars for improved safety on the roads; radionavigation-satellite applications including GPS and Galileo; TV and Internet service through terrestrial and satellite delivery; science applications such as remote sensing and radio astronomy; and countless others.