Contemporary concerns with “empirically supported treatments” emphasize the differences in outcomes that are associated with reliably delivered treatments, representing different models and theories. This approach often fails to address the fact that there is no consensus among scientists about whether there are enough differences between and among treatments to make this effort productive. There is a considerable body of data that suggests that all treatments produce very similar effects. This article reviews these viewpoints and presents a third position, suggesting that identifying common and differential principles of change may be more productive than focusing on the relative value of different theoretical models. © 2002 Wiley Periodicals, Inc. J Clin Psychol 58: 1199–1212, 2002.