Ensuring cybersecurity in digital substations is critical, particularly for systems vulnerable to cyberattacks and electromagnetic interference, with special emphasis on control and protection systems. Chapter 9 introduces the foundational principles for implementing cybersecurity measures in modern electrical power systems. It provides a historical perspective to explain the rising need to protect both primary and auxiliary substation equipment from cyber threats. Examples and catastrophic consequences of cyberattacks on power infrastructure are presented, along with a classification of cyberattacks and their mechanisms for impacting control and protection system elements, especially Intelligent Electronic Devices (IEDs). The chapter outlines vulnerability management processes, highlights the impact of cyberattacks on digital substations, and discusses the role of operational personnel and test stands in ensuring cybersecurity.