This chapter looks at what makes cyber-security a key national security issue. Section 28.2 provides background information to understand why cyberspace is insecure and what it takes to hack a system. Section 28.3 looks at different types of politically relevant cyber-operations, describing hacktivism, cyber-crime, cyber-espionage, cyber-terrorism, and cyber-war, among others. Section 28.4 puts the threat into perspective, exploring the realities of cyber-conflict. The debate has shifted from scenarios of imminent cyber-doom to the observable reality of often stealthy, sometimes disruptive operations under the threshold of war. Section 28.5 details selected protection concepts that help to reduce cyber-risks.