Robert D. Zettle, Ph.D., is associate professor in the Department of Psychology at Wichita State University. He completed his predoctoral internship at the Center for Cognitive Therapy in Philadelphia, conducted the first comparative outcome study on what is now known as ACT as part of his dissertation under the supervision of ACT founder Steven Hayes, and has published both basic and applied research relating to rule-governance, experiential avoidance, and ACT for depression for more than twenty years.