Application Security Testing (AppSec Testing) encompasses methodologies, tools, and practices for identifying vulnerabilities in software applications throughout the development lifecycle. Testing techniques range from automated static and dynamic analysis to manual code review and penetration testing, each addressing different phases from development to production. Modern AppSec integrates security testing directly into CI/CD pipelines (shift-left approach), enabling continuous vulnerability detection while reducing remediation costs through early discovery. The field emphasizes combining multiple testing approaches—white-box, black-box, and gray-box—to achieve comprehensive coverage of both code-level flaws and runtime vulnerabilities. Understanding when to apply each testing method and how to integrate them into secure SDLC practices is essential for building resilient applications in 2026.