Background: In the last decade, the clinician-patient relationship has become more of a partnership. There is growing interest in shared decision-making (SDM) in which the clinician and patient go through all phases of the decision-making process together, share treatment preferences, and reach an agreement on treatment choice. The purpose of this review is to determine the extent, quality, and consistency of the evidence about the effectiveness of SDM. Method: This is a systematic review of randomised controlled trials (RCTs) comparing SDM interventions with non-SDM interventions. Eleven RCTs met the required criteria, and were included in this review. Results: The methodological quality of the studies included in this review was high overall. Five RCTs showed no difference between SDM and control, one RCT showed no short-term effects but showed positive longer-term effects, and five RCTs reported a positive effect of SDM on outcome measures. The two studies included of people with mental healthcare problems reported a positive effect of SDM. Conclusions: Despite the considerable interest in applying SDM clinically, little research regarding its effectiveness has been done to date. It has been argued that SDM is particularly suitable for long-term decisions, especially in the context of a chronic illness, and when the intervention contains more than one session. Our results show that under such circumstances, SDM can be an effective method of reaching a treatment agreement. Evidence for the effectiveness of SDM in the context of other types of decisions, or in general, is still inconclusive. Future studies of SDM should probably focus on long-term decisions.