Abstract-Theimplementation of cyber threat countermeasures requires identification of points in a system where redundancy or other modifications are needed. Because large systems have many possible threats that may be interdependent, it is crucial that such threats be cataloged in a manner that allows for efficient representation and ease of analysis to identify the most critical threats. To address this problem, we model large system threats by conceptually representing them as a Cyber Threat Tree implemented as a directed graph known as a Multiple-Valued Decision Diagram (MDD). The cyber threat tree structure improves upon both the classical fault tree and attack tree structures by expanding the representation of possible system threats. This cyber threat tree model is incorporated into an existing MDD software package to help identify and catalog possible system threats. We have also developed a new formal language, CyTML, which is used to represent cyber threat trees.