Preparation of this manuscript was funded, in part, by a grant from Digital Equipment Corporation which is gratefully acknowledged. While we would not want to hold any of our colleagues responsible for the particular formulations we have proposed in this paper, we do want to emphasize that the thinking represented here is fundamentally a collaborative endeavor based on collaborative practice. We are particularly indebted to the members of the Interaction Analysis Laboratories at Michigan State University, Xerox PARC and the Institute for Research on Learning, especially Jeanette Blomberg, Eric Bredo, Francoise Brun-Cottan, Maureen Callanan, Bill Clancey, Penny Eckert, Fred Erickson, Shelley Goldman, Charles Goodwin, Marjorie Goodwin, Jim Greeno, Rogers Hall, Susan Irwin, Ted Kahn, Edouard Lagache, Charlotte Linde, Scott Minneman, Susan Newman, Gunther Pfaff, Britt Robillard, Jeremy Roschelle, Ron Simons, Susan Stucky, Lucy Suchman, John Tang, Debra Tatar, Randy Trigg, Etienne Wenger, Karen Wieckert, and Bob Wilks and the many guests who have presented their work there throughout the years. For detailed comments on the manuscript, some on very much earlier versions, we thank Bob Anderson, Russ Bernard, Danny Bobrow, Dede Boden, Kathy Carter, Bill Clancey, Allan Collins, David Frohlich, Rogers Hall, Ed Hutchins, Bob Irwin, Charlotte Linde, John McDermott, Moira Minoughan, William Newman, Julia Oesterle, Franca Pizzini, Ronald C. Simons, Lucy Suchman, Debra Tatar, Randy Trigg, Richard Young, and especially Jim Greeno for editorial shepherding.