This document discusses MITRE ATT&CK, which is a knowledge base of adversary behavior techniques based on real-world observations. It is free, open, and globally accessible. The document explains how ATT&CK can be used for threat intelligence, detection, adversary emulation, and assessment/engineering. It provides examples of techniques like spearphishing attachments and profiles of adversary groups like APT29. It also describes how ATT&CK can help find gaps in an organization's defenses through red team testing.