This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book takes as its starting point a methodological debate among economists about the relation of economics and psychology. It gives an overview of behavioural economics, and shows that the relationship between psychology and economics as it is featured in behavioural economics cannot be subsumed under the standard account of intertheoretic relations. Expected utility theory is a basic model of the rational economic agent, which can be extended in various ways. Procedural invariance means that people have fixed preferences over possible outcomes and that different ways of inferring these preferences will yield the same results. Violations of procedural invariance mean that features of the decision problem such as the response mode influence the decisions of people. Foad Dizadji-Bahmani, Roman Frigg and Stephan Hartmann argue that the Nagelian theory is the right analysis of intertheoretic reductive relations between theories of a different subject matter.