During carcinogenesis, precancers (premalignant lesions) are the morphologically identifiable lesions that precede invasive cancers. In theory, the successful treatment of precancers would result in the eradication of most human cancers. Despite the importance of these lesions, there has been no effort to list and classify all of the precancers. In 2001, the NCI sponsored a workshop on the classification of precancers. When considering all the possible classes of precancers, it is worth noting that not all precancers are neoplastic. In fact, precancers need not progress to cancer, and precancerous lesions often have a high rate of regression. Thus, the following five classes were adopted: 1) acquired microscopic precancers; 2) acquired large lesions with microscopic atypia; 3) precursor lesions occurring with inherited hyperplastic syndromes that progress to cancer; 4) acquired diffuse hyperplasias and diffuse metaplasias; and 5) currently unclassified entities. In this review paper, precancerous lesions of the stomach are classified and their clinical significance is described.