This chapter introduces the modeling process of SGINs. The purpose of modeling is to concretize, mathematize, and formalize the abstract process of network information services, thereby providing a model foundation for analysis and optimization (Crouch, Rosalind, and Christopher Haines, International Journal of Mathematical Education in Science and Technology 35:197–206, 2004). A good model can make the analysis more accurate, but overly concrete models may introduce significant computational complexity. Due to the involvement of more heterogeneous missions, nodes, and links, such as observation missions and data transmission missions in wide-area broadband access applications, the SGIN is more prone to modeling errors than terrestrial networks. To fix the above problems and lay the theoretical foundation for subsequent chapters about performance analysis and optimization, we start with the modeling of missions, nodes, and links, accurately characterizing the basic elements of the network, which enables us to determine their modeling processes. On this basis, the overall network modeling of the SGIN is performed from the perspectives of both single and multiple layers, which provides network-level models in terms of the relationships among layers.