1 This work was supported in part by the NSF.
2 This paper is based on lectures at the Fields Institute on June 25–27, 2003. Slides and audio are available at http://www.fields.utoronto.ca/audio/02-03/#CMI_summer_school
3 Copyright 2003, Thomas C. Hales.
4 This work is licensed under the Creative Commons Attribution License. To view a copy of this license, visit http://creativecommons.org/licenses/by/1.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.