MITRE ATT&CK (Adversarial Tactics, Techniques, and Common Knowledge) is a freely accessible knowledge base that organizes real-world attacker behaviors into a matrix of tactics and techniques. Tactics describe the “why” (e.g., Initial Access, Lateral Movement), while techniques describe the “how” (e.g., Phishing, Pass-the-Hash). The database is built from documented attacks and is continuously updated. You can use ATT&CK to prioritize detection rules in your SIEM, identify coverage gaps, and structure threat scenarios for your risk analysis. Many security products reference ATT&CK technique IDs directly in their alerts.