Second opinion is a decision-support tool for ratification or modification of a suggested treatment, by another physician. Second opinion may have a critical influence on the diagnosis, treatment and prognosis. The patient can benefit from treatment optimization and avoid unnecessary risks. The physician can benefit from less exposure to legal claims, and healthcare organizations can benefit from increased treatment, quality assurance and costs saving from unnecessary surgery and treatments. Nevertheless, injudicious use of this tool can provoke unnecessary medical costs. In recent years, many patients prefer to seek a second opinion on their disease and available treatments. Private and public insurance companies are trying to control surgery costs by urging and even demanding a second opinion before surgery. Although second opinions are common in medical practice, relatively little is known on this subject. Most of the studies reviewed in this article evaluated the clinical benefit of second opinions, the reasons patients seek a second opinion and the characteristics of these patients, as well as technological interventions to promote second opinions, and ethical or legal issues related to second opinions. Yet, there are opportunities for further studies about physicians attitudes and barriers towards second opinions, their effect on patient-physician communication and cost-effectiveness analyses of second opinions. Due to the relevance of second opinions for public heath, this review aims to summarize the current research on second opinions.