We would like to thank Sheila Coyazo, Kevin Knight, Bill Swartout, Marcelo Tallis, Milind Tambe, and Andre Valente for their comments on this paper. We would also like to thank the PROTEGE group at Stanford University, and in particular John Gennari, for creating the U-Haul domain and for making it available to us. We gratefully acknowledge the support of Defense Advanced Research Projects Agency with the contract DABT63-95-C-0059 as part of the DARPA/Rome Laboratory Planning Initiative.