With the skyrocketing number of connected devices and dramatic growth in data traffic, terrestrial networks alone cannot meet the massive and global connectivity needs in an effective and efficient manner. It is envisioned that airborne and space communication infrastructures, such as low Earth orbit (LEO) satellite constellations and unmanned aerial vehicles (UAVs), will be integrated with terrestrial networks to provision more comprehensive and three-di-mensional network connectivity, anywhere and anytime. As the fundamental information infrastructure, space-air-ground integrated networks (SAGINs) will also provision ubiquitous communication, computation, and caching to achieve high data rate, low latency, and high reliability.