Today’s analog voice-based air–ground communication system for tactical aircraft guidance is suffering from the VHF band’s increasing saturation in high-density areas. The air–ground communication infrastructure is therefore undergoing digitisation to ensure the sustainable growth of the air transportation system in the coming decades. As safety and security are strongly interrelated in aviation, strong cybersecurity is the foundation and enabler for digitalization in aviation. One of the new air-ground datalinks that shall enable this transformation is the L-band Digital Aeronautical Communication System (LDACS). It will be the primary long-range terrestrial datalink of the future IP-based aeronautical telecommunications network. In this chapter we describe the design process, draft, and the state-of-the-art cybersecurity architecture for LDACS.