Securing critical infrastructures and their associated mission-critical communications is of utmost importance because they are the underpinnings of our modern society. From energy to transportation to public safety to mining, the advanced communications networks used by critical infrastructure operators carry data generated by electric grid IEDs, sensors, meters, surveillance cameras, LMR and 4G/LTE systems installed across the networks. These applications support unprecedented levels of real-time monitoring and situational awareness, leading to improved operations efficiency, reliability, resiliency and safety. At the same time, cyber attacks increasingly threaten to disrupt critical infrastructure operations, making adoption of a defense-in-depth approach vital. This paper examines how critical infrastructure operators can identify, address and mitigate these threats with best practices and methodologies centered on the ITU-T X.805 security framework using Nokia security capabilities – specifically an in-depth network security solution to address evolving security requirements.