On May 22, 2001, the Payment Cards Center of the Federal Reserve Bank of Philadelphia sponsored a workshop on the role of interchange fees in the credit card industry. The workshop discussion, led by Dr. David Humphrey of Florida State University, explored the framework behind interchange fees.2 It then focused on several open questions that have arisen within both the academic research and international regulatory communities about interchange's impact on payment system efficiency. The paper that follows is supplemented by additional research and provides background information on interchange, a transaction-level overview of the credit card system that results in the extraction of interchange, and a summary of Dr. Humphrey's comments.