The authors would like to thank Paul Resnick for suggesting this topic and providing detailed comments, as well as Tom Malone for his feedback and encouragement. In writing this chapter, we have benefited greatly from discussions with Brian Pentland, Kevin Crowston, Chris Dellarocas, Fred Luconi, and Charley Osborn. Bob Halperin, Lorin Hitt, Kazuo Okamura, John Quimby, Ignascio Silva-Lepe, Tor Syvertsen, Marshall Van Alstyne, and the late Cheng Hian Goh also provided helpful comments. This research was sponsored by the Center for Coordination Science at the Massachusetts Institute of Technology and by the National Science Foundation (No. IRI-9224093).