As part of IDEI's research into network economics, we have formally developed theory to analyze markets that act as platforms connecting disparate groups. The value of a multi-sided market platform to each group it serves increases as the size of every other group grows. A stock exchange matches buyers and sellers, a television station matches creators of programming and viewers, and a payment card association matches merchants and cardholders. The IDEI Payment Card Center of Research Excellence (Payment Card CORE) provides a repository for scholarly work on multi-sided markets, with a particular emphasis on the economics of payment cards. The structure of multi-sided markets, tools for optimizing pricing to each side of a multi-sided market and the implications for competition policy are all addressed.