"Truth is the essence of science," and "peer review" is a key element in assuring the "truth". Peer review is particularly important with regard to diagnostic pathology in bioassays used for regulatory purposes and approval of drugs and medical devices for at least 2 reasons: (a) If a study is designed correctly and conducted properly, the bottom-line results rely almost entirely on pathological interpretations, and (b) diagnostic pathology is a subjective science that relies on the training and experience of the pathologist and therefore is subject to individual bias. "Bias" can be introduced during the collection and preparation of pathological materials, use of inconsistent terminology during the pathology evaluation and interpretation of the results. Peer review can help in reducing this bias. However, peer review is also subject to bias by knowledge of treatment groups, selection of inappropriate slides for review, in the type of question asked of the reviewers, and in the selection of the reviewers. When done correctly, pathology peer review can be an effective and important part of a process to assure the results of a study, but when done improperly it can cloud the issue and have a negative impact on the credibility of the study.