A Practice Note providing an overview of what hacking and network intrusions are, how they work, and how to prevent, detect, and respond to these cyberattacks. This Note discusses common cybersecurity vulnerabilities in information technology (IT) and operational technology (OT) systems and the resulting attacks that often occur, including denial-of-service (DoS) and distributed denial-of-service (DDoS) attacks, insider attacks, data theft (exfiltration), sabotage and extortion attacks, misuse, and advanced persistent threats (APTs).