The Hill Interaction Matrix (HIM) is a behavioral rating system that measures the therapeutic quality of group participant interactions. Accordingly, the HIM articulates an explicit value system for what is deemed "therapeutic" in group member interactions. This value system emerged from a rich theoretical heritage of group dynamics, which is summarized first. In the next section we briefly describe the most recent version of the HIM and how the value system was used to develop the two dimensions of member behavior (i.e., content and work style) rated by the Hill Interaction Matrix-Statement by Statement (HIM-SS). We then focus on the correspondence between the theory and measure, examining the psychometric strengths and limitations of the HIM-SS as well as findings from previous research. After applying the HIM-SS to a common transcript, we close with a discussion on the unique contribution made by HIM to both the group literature and the training of neophyte group therapists.The development of the HIM extended over two decades (1950s-We acknowledge the contribution of William Fawcett Hill, who furnished historical and contextual background, and Priscilla S. Hill, who provided consultation on the history, development, and recent use of the Hill Interaction Matrix.