Situation awareness (SA) is a person's awareness about the world around them. SA resonates with many researchers and practitioners, motivating the development of many theories and measures. This chapter summarizes the diverse SA research to highlight the strengths of individual SA theories and describes how to measure SA according to well-established techniques. We thus recommend that SA applications should be based on theories and measurement methods suitable to accounting for the unique characteristics of the domain. To illustrate the practical significance of SA, we present a case study of developing and applying an SA measure to assess user interfaces for nuclear power plant control room. Future research is expected to focus on modeling awareness of automation and employing biosensors for improving measurements of SA.