The cash-in-advance constraint, also known as the Clower constraint after American economist Robert W. Clower,[1] is an idea used in economic theory to capture monetary phenomena. In the most basic economic models (such as the Walras model or the Arrow–Debreu model) there is no role for money, as these models are not sufficiently detailed to consider how people pay for goods, other than to say everyone has a budget constraint. To be able to say anything about the money supply, inflation, monetary policy and so on, economists must therefore introduce additional assumptions into their models. One possibility, and the more popular one, is to introduce a cash-in-advance constraint i.e. a requirement that each consumer or firm must have sufficient cash available before they can buy goods.[2] An alternative assumption would be a 'Money-in-the-Utility-Function' assumption pioneered by Miguel Sidrauski, which states that people have a tendency to hold a certain amount of cash because they derive utility from holding it. Without these (or similar) assumptions economic theory would find it difficult to explain why people carry around a good (money) which takes up space in their wallet, can't be consumed and does not earn any interest.