BACKGROUND: In the last few decades, therapeutic horse-riding has become recognized as a progressive form of therapy, particularly for people with disabilities. Although there is a substantial amount of literature that supports the physical benefit of therapeutic riding, only anecdotal evidence exists in relation to its psychological benefits. OBJECTIVE: The purpose of this article is to develop hypotheses about the mechanisms by which therapeutic riding might have a beneficial psychological effect. These hypotheses can then be tested, leading to a more detailed knowledge base. DATA SOURCES: PsychINFO, MEDLINE, PROQUEST, Scopus, Web of Science, and CINAHL. STUDY SELECTION: Data sources were searched for studies that (a) were related to the psychological effects of therapeutic horse-riding, (b) focused exclusively on therapeutic horse-riding, (c) described, explicitly or implicitly, the mechanism by which therapeutic riding had a beneficial psychological effect. Studies were limited to those published between 2008 and 2012. DATA EXTRACTION: Data were extracted by two authors independently. DATA SYNTHESIS: Thirty articles met the inclusion criteria. Three potential hypotheses emerged from the literature, namely, (1) the psychological benefits of therapeutic riding are actually unrelated to the horse, (2) the horse provides a particularly positive context within which psychological gains are facilitated, and (3) the horse itself has specific therapeutic qualities that bring about unique changes not otherwise likely to occur. CONCLUSION: The challenge for researchers in this area is to design studies that adequately test these competing hypotheses.