This chapter reviews situation awareness and decision making, and discusses the types of data needed to assess decision making when conducting error investigations. An operator with deficient initial situation awareness may have difficulty obtaining more accurate situation awareness subsequently. Operators perform situation assessment to understand the system state and its operating environment. Operators with deficient or inaccurate situation awareness have difficulty interpreting system-related information and are likely to commit errors. Investigators sought to explain how the crew failed to obtain situation awareness with regard to the engine that had failed, despite the presence of engine status cues and sufficient time to comprehend the given information. Because of the influence of decision making on operator performance, investigators should be familiar with the process and its role in error. The quality of situation awareness directly affects the quality of subsequent decision making.