Since 2003, the Oral History Project Team has conducted interviews with individuals who have made particularly noteworthy contributions to the theory and practice of evaluation. In 2013, Mel Mark, Valerie Caracelli, and Miles McNall sat with Thomas Cook in Washington, D.C., during the American Evaluation Association (AEA) annual conference. The interview was taped and subsequently transcribed verbatim into a 37-page document. The authors edited the transcript for clarity and length. Cook reviewed and approved the final product prior to its submission to "American Journal of Evaluation." Thomas D. Cook was the Joan and Sarepta Harrison Chair of Ethics and Justice at Northwestern University, where he is professor emeritus of sociology, psychology, education, and social policy. Cook is well known for his work on social science methods, especially for his books on quasi-experiments with Donald Campbell and William R. Shadish. In this contribution to the oral history project, Cook describes how he went from working on a PhD in German literature to working in the Department of Psychology at Northwestern, where Donald Campbell provided his entrée to program evaluation. Cook describes his initial evaluation project, the early history of the Evaluation Research Society (ERS), the writing of the Cook and Campbell book on quasi-experimentation, Cronbach's alternative view of validity, mixed methods and critical multiplism, and more. [This article was written with the Oral History Project Team, which consists of Robin Lin Miller, Jean A. King, Melvin M. Mark, and Valerie Caracelli.]