I n a modern economy, a large fraction of payments for goods and servicesinvolve the services of one or more banks. The provision of paymentservices is, in fact, one of the distinguishing characteristics of banks. A bank-intermediated payment instrument, such as a check, typically commu-nicates instructions to the buyer’s bank to make payment to the seller or the seller’s bank. Often, then, we think of payment services as being bundled with the deposit services provided by banks, although this is not always the case. Credit cards, for instance, involve payments by the card-issuing bank, at which the cardholder need not hold deposits. Still, many payment services do arise naturally as byproducts of holding deposits with a bank, and some auth...