The assessment of psychotherapies, for reasons related both to the respect of public health rules and to the improvement of our knowledge and our practices, has become an unavoidable stage. The effectiveness of psychotherapies, and particularly of Cognitive and Behavioral Therapies (CBT), is now widely documented in the treatment of depressive episodes and in the prevention of relapses. Recent work focuses on improving care to further optimize the gain in relapse prevention. It is within this framework that the MBCT approach takes its place. Studies have shown that joining an MBCT approach further improves the gain in relapses after CBT to which the patient had responded (Vittengl et Jarrett, 2015). On the other hand, the question of why they work, under what conditions, for which particular individuals and the identification of the mechanisms and processes of change are still very current. In order to do this, the evaluation of the effect of a practice in MBCT used both direct measures (self-questionnaires) and more indirect measures of change (neuropsychological tests, psychological measures, brain imaging, etc.). The purpose of this paper is to review the results of these measures, the results they have produced and the hypotheses they have raised in the international literature on unipolar disorder.