This article describes the results of comparisons of digitally scanned whole slide images (WSIs) and glass microscope slides for diagnosis of tissues under peer review by the National Toxicology Program. Findings in this article were developed as a result of the data collected from 6 pathology working groups (PWGs), 1 pathology peer review, and survey comments from over 25 participating pathologists. For each PWG, 6–14 pathologists examined 10–143 tissues per study from 6- and 9-month perinatal studies and 2-year carcinogenicity studies. Overall it was found that evaluation of WSIs is generally equivalent to using glass slides. Concordance of PWG consensus diagnoses based upon review of WSIs versus glass slides ranged from 74% to 100% (median 86%). The intra- and interobserver diagnostic variation did not appear to influence the conclusions of any study. Based upon user opinions collected from surveys, WSIs may be less optimal than glass slides for evaluation of subtle lesions, large complex lesions, small lesions in a large section of tissue, and foci of altered hepatocytes. These results indicate that, although there may be some limitations, the use of WSIs can effectively accomplish the objectives of a conventional glass slide review and definitely serves as a useful adjunct to the conduct of PWGs.