Payment systems are a vital part of the financial infrastructure and market economies, and by functioning reliably and efficiently they contribute to general stability and efficiency in the economy. A core task for payment systems is to facilitate the settlement of monetary liabilities arising from the business activities of entities in markets for goods and in financial markets. Users of payment systems expect their payments to be carried out securely, quickly and efficiently, allowing debtors to settle their liabilities to creditors via the systems. Payment system can be useful and that, in the absence of private money, central bank money and central bank lending through the discount window can deliver welfare benefits over and above what can be accomplished with a private interbank money market. The deep insights gained from this framework can be used to help shape answers to questions about the future design of payment systems, though their existing framework is perhaps as yet too primitive to deliver sharp and robust policy insights.