The lecture gives a broad introduction to network security, including foundations of cryptography, message integrity, authentication, privacy and anonymity, application layer security, secure network protocols, security in the physical layer, as well as broader aspects security aspects related to reliability and safety. It also discusses relevant topics in various application domains, such as (i) security in next generation mobile networks, (ii) satellite network security; (iii) security in the compute continuum of IoT, edge and cloud computing; (v) security functions within the network management; (vi) physical layer security in optical and wireless networks.