INTRODUCTION: The therapeutic alliance in psychotherapy refers to the quality of the relationship between therapist and patient, as well as their agreement upon the aims and tasks of the treatment. This concept is widely used in clinical settings and, since the validation of a number of instruments, has become the focus of much research. However, the quantity of experimental evidence now available impedes the dissemination of these findings among clinicians in a concise manner. LITERATURE FINDINGS: The present paper aimed to provide a review of the literature reporting experimental evidence focusing on the therapeutic alliance. In order to do this, findings were organized around four main issues: the relationship between the therapeutic alliance and clinical outcome; the evolution of the therapeutic alliance over time; the mechanism of action by which the therapeutic alliance encourages change, and finally the factors that have been shown to influence the creation and evolution of the alliance. DISCUSSION: This review reveals that the therapeutic alliance is to this day one of the most investigated factors of psychotherapy. The therapeutic alliance enables change to take place during psychotherapy and, along with a number of specific factors, appears to be important for clinical outcome. Over time, the therapeutic alliance has been seen to evolve and several patterns have been identified. A number of characteristics inherent to the patient, the therapist, and even their interaction have been reported to influence the development and strength of the therapeutic alliance. Several hypotheses have been put forward to account for the mechanism by which the therapeutic alliance encourages change. CONCLUSION: The area of research regarding the therapeutic alliance remains dynamic. Despite the present body of knowledge surrounding this concept, a number of research questions await further investigation.