In the field of gut endocrinomas (carcinoids and relevant neoplasms), several classifications have been internationally accepted and utilized at varying frequency. The basis of the concepts regarding gut endocrinomas from which these classifications have been proposed were drawn from the different aspects. These included embryology, histologic growth patterns, histochemistry including silver impregnations, electron microscopic morphology of endocrine secretory granules, endocrine cell types and histologic morphology along with functional characteristics, supplemented by immunohistochemical features. Due to continuous progress being made in this particular field of research and the many new discoveries made by pioneering investigators, the concepts of gut endocrinomas have been modified and revised during such long history of the research activities. This study aims to re-evaluate these classifications in relation to the concepts of gut endocrinomas, and to select and supply rearranged classifications that may be easily utilized for practical purposes. This study also proposes a comprehensive overview of histogenesis in the gut endocrinoma group. This consists of typical carcinoids and their atypical variants taken in relation to the carcinomatous group including ordinary carcinomas and their variants with endocrine elements. Special emphasis is given to the point that there is a gradual transition, one without a definite boundary between these tumors.