Welcome to the IEEE JSAC special issue on Spectrum Sharing and Aggregation for Future Wireless Networks. The conception of this special issue is motivated by the following observations: the ever-increasing penetration of both the mobile Internet and of the Internet-of-things is gradually clogging up the most valuable spectral bands available in the sub-2 GHz frequency range for future wireless networks. Hence there is an urgent need for improved spectrum exploitation to satisfy this demand. It is expected that the wireless tele-traffic will continue to grow quite dramatically in the ensuing years, hence further widening the spectrum-supply versus demand gap. In order to mitigate this gap, spectrum sharing and aggregation have been well recognized as promising approaches, which led to rapid advances by harnessing a large cross-section of the research community. Nonetheless, there are numerous unsolved technical challenges. This special issue aims for reporting on some of these cutting-edge advances in spectrum sharing and aggregation, whilst opening new avenues of research in this area.