Cyber security is widely viewed as a matter of pressing national importance. Many elements of cyberspace are notoriously vulnerable to an expanding range of attacks by a spectrum of hackers, criminals, terrorists, and state actors. For example, government agencies and private-sector companies, both large and small, suffer from cyber thefts of sensitive information, cyber vandalism (e.g., defacing of websites), and denial-of-service attacks. The nation's critical infrastructure, including the electric power grid, air traffic control system, financial systems, and communication networks, depends extensively on information technology for its operation. National policymakers have become increasingly concerned that adversaries backed by considerable resources will attempt to exploit the cyber vulnerabilities in the critical infrastructure, thereby inflicting substantial harm on the nation. Numerous policy proposals have been advanced, and a number of bills have been introduced in Congress to tackle parts of the cyber security challenge. This book is designed to serve as the textbook for a semester course devoted to cyber security. It is focused on helping students acquire the skills sought in the professional workforce.