In their everyday operations, controllers are often placed in situations that are either unfamiliar or filled with uncertainty. Without an understanding of the situation, controllers cannot take timely and adequate actions. Making sense of critical situations is difficult, especially when controllers are faced with abundant, conflicted, or limited information. In recent years, the expansion of information technology has increased the amount of information presented to controllers without any assistance on how to make sense of the situation or how to anticipate future trends of the situation. Air Traffic Control is a complex and dynamic environment that requires controllers to attend to multiple events, register fast changing datadiagnose system failures, and resolve conflicts while maintaining resources to handle traffic and, above all, make sense of evolving scenarios. Sensemaking has been viewed as a retrospective activity of individuals and teams bounded by organizational rules and constraints Sensemaking has implications for the design of training curricula and decision support systems, especially for major ATM system-wide interventions (e.g., SESAR and NextGen). In this chapter, a more elaborate discussion is provided of individual and team sensemaking functions because of the importance of managing modern ATM systems that present high levels of uncertainty and complexity.