Motivational Interviewing (MI) and its offshoots such as the Drinker's Check-up and Motivational Enhancement Therapy have become enormously popular in scientific and addiction treatment circles over the past decade. This article first addresses the reasons for this popularity and suggests that it is due to presumed cost-effectiveness, relevance to interventions in non-treatment-seeking populations, and the essential nature of addictions as motivational disorders. The article then goes on to examine evidence for the effectiveness of MI in both non-treatment-seeking and treatment-seeking samples, taking account of published systematic reviews and, particularly, the findings of Project MATCH. The relevance of possible findings from the UK Alcohol Treatment Trial is also considered. The article also looks at mechanisms of change that may account for the way MI works. It ends with a list of conclusions bearing on the effectiveness of MI, the possible roles in treatment that it can serve, the client groups for whom it may be especially appropriate, and what further research is needed.