Peer review is generally considered the cornerstone of the scientific control system. Hence it is critical that peer review works well. The empirical finding that reviewers often disagree among themselves is the starting point for an analysis of peer review. Depending on the reasons for such disagreements, I argue that disagreement, as well as agreement, among reviewers can have both positive and negative effects for science. The empirical research on peer review is analyzed according to a categorization of review objects (manuscripts or research applications) and review outcomes (approval or rejection) and draws on psychological judgment and decision-making research. Moreover, bias in peer review is scrutinized. The conclusion offers implications for the peer review system and for scientific research.