'Medical science can only flourish in a free society and dies under totalitarian repression.' (1) Peer review post-publication is relatively easy to define: when the world decides the importance of publication. Peer review pre-publication is what the scientific community frequently means when using the term 'peer review'. But what it is it? Few will agree on an exact definition; generally speaking, it refers to an independent, third party scrutiny of a manuscript by scientific experts (called peers) who advise on its suitability for publication. Peer review is expensive; although reviewers are unpaid, the cost in time is enormous and it is slow. There is often little agreement among reviewers about whether an article should be published and peer review can be a lottery. Often referred to as a quality assurance process, there are many examples of when peer review failed. Many will be aware of Woo-Suk Hwang's shocking stem cell research misconduct at Seoul National University. (2) Science famously published two breakthrough articles that were found subsequently to be completely fabricated and this happened in spite of peer review. Science is not unique in making this error. However, love it or hate it, peer review, for the present time at least, is here to stay. In this article, Philippa Benson, Managing Editor of Science Advances (the first open access journal of the American Association for the Advancement of Science), discusses the merits of peer review. Dr Benson has extensive experience in the publishing world and was Executive Director of PJB Consulting, a not-for-profit organisation supporting clients on issues related to converting to full electronic publishing workflows as well as challenges working with international authors and publishers. Her clients included the Public Library of Science journals, the American Society for Nutrition and the de Beaumont Foundation. She recently co-authored a book, What Editors Want: An Author's Guide to Scientific Journal Publishing (University of Chicago Press), which helps readers understand and navigate the publishing process in high impact science and technical journals. Her master's and doctorate degrees are from Carnegie Mellon University. JYOTI SHAH Commissioning Editor