The clinical method of motivational interviewing (MI) evolved from the person-centered approach of Carl Rogers, maintaining his pioneering commitment to the scientific study of therapeutic processes and outcomes. The development of MI pertains to all 3 of the 125th anniversary themes explored in this special issue. Applications of MI have spread far beyond clinical psychology into fields including health care, rehabilitation, public health, social work, dentistry, corrections, coaching, and education, directly impacting the lives of many people. The public relevance and impact of clinical psychology are illustrated in the similarity of MI processes and outcomes across such diverse fields and the inseparability of human services from the person who provides them, in that both relational and technical elements of MI predict client outcomes. Within the history of clinical psychology MI is a clear product of clinical science, arising from the seminal work of Carl Rogers whose own research grounded clinical practice in empirical science. As with Rogers’ work 70 years ago, MI began as an inductive empirical approach, observing clinical practice to develop and test hypotheses about what actually promotes change. Research on MI bridges the current divide between evidence-based practice and the well-established importance of therapeutic relationship. Research on training and learning of MI further questions the current model of continuing professional education through self-study and workshops as a way of improving practice behavior and client outcomes.