Peer review in all its form plays an important role in ensuring the integrity of the scholarly record. The process depends to a large extent on trust, and requires that every one involved behaves responsibly and ethically. Peer reviewers playa central and critical part in the peer-review process, but too often come to the role without any guidance and maybe unaware of their ethical obligations. The COPE ethical Guidelines for Peer Reviewers set out the basic principles and standards to which all peer reviewers should adhere during the peer-review process. It is hoped they will provide helpful guidance to researchers, be reference for journals and editors in guiding their reviewers, and act as an educational resource for institutions in training their students and researchers.