Background: There is increasing evidence claiming the effectiveness of motivational interviewing (MI) in promoting behavioural change. However, ongoing changes to MI theory and practice have implications for its transferability, accessibility and for the validity of previous findings. Lack of practice consistency may make its effectiveness difficult to evaluate. Aims: This paper explores the complexity of MI and issues in the development of evidence-based practice in delivery, before describing issues related to practitioner application. Method: Theoretical and practice developments over the last 30 years are reviewed under the headings theory, practice and efficacy. Specifically, developments across the three editions of the core MI texts are examined. Results: Findings from the literature suggest a lack of theoretical stability and practice integrity, with recent fundamental changes to the underpinning structure of MI. Issues relating to the transferability and acquisition of MI skills, consistency of delivery and mechanisms underlying change are discussed. Conclusions: The authors call for greater theoretical stability, more transparency over how developments are based on theoretical principles and empirical outcomes, and clearer guidance about how this informs practice development and delivery of MI.