The purpose of this atlas is to provide surgical pathologists with ready expert reference material most helpful in their daily practice. The lesions described relate principally to medical non-neoplastic conditions as exemplified by our first three Fascicles on endocrine, pulmonary, and skin diseases. Many of these lesions represent complex entities and when appropriate, we have included contributions from internists, radiologists, and surgeons. This has led to some increase in the size of the monographs but the emphasis remains on diagnosis by the surgical pathologist.