The origins of the term ‘schema’ are traceable to the writings of Plato, Aristotle and Kant (Marshall, 1995). The term was popularised by Bartlett in the 1930s, although Schema Theory, as it is known today, evolved after the onset of the Cognitive Revolution in the 1980s (Anderson, 1995). Schema Theory has proved contentious since the mid-1900s, when it emerged in the mainstream psychological literature. Schema Theory has seen its fair share of criticism, for example much debate surrounds the legitimacy of the term Schema Theory. However, the theory continues to generate research and in recent years has been influential in many areas of Ergonomics, including human-computer interaction (Chalmers, 2003), tool use (Baber, 2006), military applications (e.g. Stanton et  al., 2006; Stewart et al., 2008; Salmon et al., 2009a) and a variety of transport domains including road (Hole, 2007; Walker et al., 2011; Salmon et al., 2014a), rail (Stanton and Walker, 2011; Salmon et al., 2013a) and aviation (Plant and Stanton, 2012a; Chapter 3). This chapter provides an overview of the development of Schema Theory by discussing what constitutes a schema and the pioneers of the theory. Section 2.2 discusses the criticisms that have been levelled against the theory. The chapter is intended to demonstrate how Schema Theory has influenced research questions and been applied in Ergonomics as a theoretical foundation, most notably in the fields of situation awareness (SA), decision-making and error research. In doing so, Section 2.3 demonstrates how the criticisms of Schema Theory can and have been addressed so that the value of Schema Theory can be realised in Ergonomics research and practice. Furthermore, potential future directions for Schema Theory are discussed in Section 2.4. The chapter is concluded in Section 2.5, where it is argued that the value of Schema Theory lies in its inclusion in the perceptual cycle model (PCM), which models the interaction between people and their environment, thereby providing a systemic explanation for decision-making and action. This will form the theoretical underpinnings for the remainder of the research in this book.