Multiple services we use every day, from Zoom to cryptocurrency wallets, rely on large-scale networked systems. These systems consist of a complex series of interdependent components and control algorithms, which make their management particularly challenging. In effect, we often observe disturbances such as cross-layer security vulnerabilities and unpredictable interferences across seemingly independent applications. In this course we revisit a few fundamental network topics (such as routing, monitoring, and congestion avoidance) aiming at revealing and addressing their performance and security implications.