With its dispersed and dynamic features, the cloud-edge-terminal network architecture offers customers effective and adaptable network services, satisfying the increasing demand for network services. However, this architecture also introduces additional security risks. This chapter will provide a comprehensive overview of the cloud-edge-terminal network architecture, briefly outline the specific components and network services provided by each layer, and conduct a thorough analysis of the network risks faced by each layer. Additionally, this chapter will explore Mobile Target Defence (MTD) technology, a novel defence technique employing proactive defence strategies, whose emergence offers a feasible technical approach for providing reliable security protection for cloud-edge-terminal networks. We will outline the fundamental design principles of MTD and, from the perspective of network operation mechanisms, classify mainstream MTD solutions into active virtual migration, proxy shuffling, and network property mutation. We will provide a detailed introduction to the defence principles of these three MTD solutions and list specific system cases. This chapter provides the background information readers need to comprehend the book’s content by introducing cloud-edge-terminal networks and MTD technology.