The monograph summarizes the results of many years of research by the authors and the data of the modern literature on the pathological anatomy of chronic diseases of the stomach, small and large intestine, and anal canal. The focus is on biopsy diagnosis of these diseases. Data on the normal structure of the stomach, intestines and anal canal are also presented. All sections of the book are written on a nosological principle. They set out the latest ideas about the etiology, pathogenesis and morphological substrate of diseases. The monograph is addressed to pathologists, gastroenterologists, as well as therapists and surgeons.