With the rapid advancement of information and communications technology, the Space-Ground Integrated Network (SGIN) has emerged as a pioneering architecture blending satellite and terrestrial networks. SGINs aim to extend network coverage and provide ubiquitous communication services. While terrestrial networks ensure reliable coverage in urban areas and hotspots, satellite networks cover remote regions, oceans, and airspace applications. Beyond basic signal coverage, SGINs deeply integrate satellite and terrestrial networks. This chapter discusses the background and evolution of SGINs, including conceptualization, architecture, and typical application scenarios. We investigate typical performance metrics, including coverage, access, bearer, service, and system performance. We explore issues in SGIN modeling, analysis, and optimization, and introduce fundamental modeling and analysis techniques, which are the fundamental works for subsequent chapters. This chapter delves into the background and development of SGINs, including their pivotal role in communications. We also discuss performance metrics and contemporary technologies.