Exploring the influence and application of Campbellian validity typology in the theory and practice of outcome evaluation, this volume addresses the strengths and weaknesses of this often controversial evaluation method and presents new perspectives for its use. Editors Huey T. Chen, Stewart I. Donaldson and Melvin M. Mark provide a historical overview of the Campbellian typology adoption, contributions and criticism. Contributing authors propose strategies in developing a new perspective of validity typology for advancing validity in program evaluation including * Enhance External Validity * Enhance Precision by Reclassifying the Campbellian Typology * Expand the Scope of the Typology The volume concludes with William R. Shadish's spirited rebuttal to earlier chapters. A collaborator with Don Campbell, Shadish provides a balance to the perspective of the issue with a clarification and defense of Campbell's work. This is the 129th volume of the Jossey-Bass quarterly report series New Directions for Evaluation, an official publication of the American Evaluation Association.