Facing the needs of social development beyond 2030, mobile communication networks need to provide more comprehensive three-dimensional coverage, and satellite networks will play a more important role. Therefore, 6G will be an integrated network deeply integrating satellite networks and terrestrial networks, providing more ubiquitous, intelligent, secure, and trustworthy public mobile information basic service capabilities. Compared with terrestrial mobile communication systems, the satellite communication system has the advantages of wide coverage, large communication capacity, small terrain impact, high flexibility, and adaptability to various services. Based on an overview of the development state of integrated Satellite-Terrestrial networks (ISTNs), this chapter looks forward to the development path of the ISTN, and analyzes several key technologies adopted by the ISTN for 6G.