What is defense in depth?

Defense in depth (also called the layered security approach) is a security strategy that uses multiple independent layers of controls so that if one layer is breached, others still protect the system. Inspired by military strategy (concentric defensive rings). Layers include: Perimeter security (firewalls, WAF, DDoS protection), Network security (network segmentation, VLANs, IDS/IPS), Host security (hardening, endpoint protection, patch management), Application security (input validation, authentication, SAST/DAST), Data security (encryption at rest/transit, DLP), Identity security (MFA, least privilege, PAM), Physical security (data center access controls), and Monitoring/response (SIEM, SOC, incident response). No single control is perfect; depth ensures multiple failures must occur simultaneously for a successful breach.