An Overview of the KL-ONE Knowledge Representation System**The history of the ideas in KL-ONE is too complex to recount here, and the number of contributors to this whole area of work makes it inevitable that we will forget to mention many who deserve to be acknowledged. At the very least, we would like to extend our thanks to these people, who have been intimately involved with KL-ONE, KL-TWO, and Krypton: Danny Bobrow, Rusty Bobrow, Phil Cohen, Richard Fikes, Mike Freeman, Jeff Gibbons, Victoria Gilbert, Brad Goodman, Norton Greenfeld, Austin Henderson, David Israel, Henry Leitner, Hector Levesque, Bob Lingard, Tom Lipkis, Bill Mark, Peter Patel-Schneider, Candy Sidner, Mark Stefik, Marc Vilain, David Wilczynski, Bill Woods, Martin Yonke, and Frank Zdybel.This research was supported in part by the Defense Advanced Research Projects Agency under Contract N00014-77-C-0378. Views and conclusions contained in this paper are the authors' and should not be interpreted as representing the official opinion or policy of DARPA, the U.S. Government, or any person or agency connected with them.Correspondence and requests for reprints should be sent to Ronald J. Brachman, AT&T, Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974.