Frequently, phenotypes of new mouse models of lung cancer have no precedents or are arbitrarily attributed according to incongruent human and mouse classifications. To address these issues, a panel of human, veterinary, and experimental pathologists performed a comparative evaluation of mouse and human proliferative lung lesions, and recommended a new practical classification scheme. This classification should help investiga-tors and pathologists in their characterization of new mouse models, as well as stimulate further research aimed at a better understanding of proliferative lesions of the lung. The cover features a representative H&E section of carcinoma of the mouse lung. For details, see the article by Nikitin et al. on page 2307 of this issue.