Sections of this chapter are derived from work done jointly with Jim Short, Steve Sawyer, and Rolf Wigand. The chapter was greatly improved by comments from the track chair, associate editor, and two anonymous reviewers. The work has been partially funded by the National Science Foundation, Grant IIS 97-32799 and by a grant from the Offce of the Dean of the School of Information Studies, Syracuse University. This version of the chapter was edited to fit the page restrictions of the conference. A complete version is available from the author.