The work described in this book was first presented at the Fifth Workshop on Genetic Programming, Theory and Practice, organized by the Center for the Study of Complex Systems at the University of Michigan, Ann Arbor, 17-19 May 2007. The goal of this workshop series is to promote the exchange of research results and ideas between those who focus on Genetic Programming (GP) theory and those who focus on the application of GP to various realworld problems. In order to facilitate these interactions, the number of talks and participants was small and the time for discussion was large. Further, participants were asked to review each other’s chapters before the workshop. Those reviewer comments, aswell as discussion at theworkshop, are reflected in the chapters presented in this book. Additional information about theworkshop, addendums to chapters, and a site for continuing discussions by participants and by others can be found at http://cscs.umich.edu/gptp2007.