The article reviews the development and theoretical underpinnings of motivational interviewing (MI), a widely-disseminated clinical method for promoting behavior change. The authors highlight the convergence of scientific research and practical application in shaping MI, emphasizing its effectiveness in various therapeutic contexts. They propose an emergent theory of MI, focusing on two key components: a relational component involving empathy and a therapeutic spirit, and a technical component involving the differential evocation and reinforcement of client change talk. The article discusses the importance of therapist training and the impact of therapist and client responses during treatment sessions on post-treatment outcomes. It also explores the role of MI in combination with other active treatments, noting its synergistic effects. The authors review empirical evidence supporting the hypothesized causal chain model linking therapist training, therapist-client interactions, and client outcomes. They discuss the challenges and limitations of MI, including variability in outcomes across and within studies, and the need to understand the mediators and moderators of its efficacy. The article concludes by summarizing the evidence-based nature of MI, its broad applicability, and the emerging testable theory of its mechanisms of action.The article reviews the development and theoretical underpinnings of motivational interviewing (MI), a widely-disseminated clinical method for promoting behavior change. The authors highlight the convergence of scientific research and practical application in shaping MI, emphasizing its effectiveness in various therapeutic contexts. They propose an emergent theory of MI, focusing on two key components: a relational component involving empathy and a therapeutic spirit, and a technical component involving the differential evocation and reinforcement of client change talk. The article discusses the importance of therapist training and the impact of therapist and client responses during treatment sessions on post-treatment outcomes. It also explores the role of MI in combination with other active treatments, noting its synergistic effects. The authors review empirical evidence supporting the hypothesized causal chain model linking therapist training, therapist-client interactions, and client outcomes. They discuss the challenges and limitations of MI, including variability in outcomes across and within studies, and the need to understand the mediators and moderators of its efficacy. The article concludes by summarizing the evidence-based nature of MI, its broad applicability, and the emerging testable theory of its mechanisms of action.