Without cybersecurity, there would be no national security. Facing the reality that software and hardware vulnerabilities are inevitable and attacks with unknown patterns are unavoidable, traditional defense is insufficient. This book introduces the core ideas, defense model and framework of the built-in active defense. It starts by analyzing the inherent attack mechanisms and attack paths and finding security weaknesses exploited by attackers. It then, with the security requirements being a primary design consideration, redesigns and transforms existing technologies such that they are able to effectively suppress, detect, contain, and eliminate security threats caused by their internal vulnerabilities. The built-in active defense makes systematic breakthroughs in the overall defense strategy, moving from external to built-in, and from reactive to proactive. It essentially changes information technology with security enhancements to secure information technology. It has great practical significance, broad scientific and technological importance, and profound strategic influence.