This large-scale international study measures the attitudes of more than 4,000 researchers toward peer review. In 2009, 40,000 authors of research papers from across the globe were invited to complete an online survey. Researchers were asked to rate a number of general statements about peer review, and then a subset of respondents, who had themselves peer reviewed, rated a series of statements concerning their experience of peer review. The study found that the peer review process is highly regarded by the vast majority of researchers and considered by most to be essential to the communication of scholarly research. Nine out of 10 authors believe that peer review improved the last paper they published. Double-blind peer review is considered the most effective form of peer review. Nearly three quarters of researchers think that technological advances are making peer review more effective. Most researchers believe that although peer review should identify fraud, it is very difficult for it to do so. Reviewers are committed to conducting peer review in the future and believe that simple practical steps, such as training new reviewers would further improve peer review.