Imposex, the superimposition of male-type genital organs (penis and vas deferens) on female gastropods, is cause-specific and induced by low concentrations of certain organotins, such as tributyltin (TBT) and triphenyltin (TPT) from antifouling paints. Reproductive failure may be observed at severely affected stages. The history of imposex study and the legislation of organotins are summarized. The present statuses of gastropod imposex and organotin pollution in Japan, together with endocrine disruption observed in abalones, are also summarized, on the basis of the results of the field studies of the rock shell (Thais clavigera) conducted at 174 locations along the Japan coast between January 1999 and November 2001, the histopathological and analytical chemical studies of the ivory shell (Babylonia japonica), and both field and laboratory studies of the abalone (Haliotis madaka and H. gigantea). The fundamental knowledge of the endocrinology of gastropod mollusks is briefly described. Four hypotheses, such as that of aromatase-inhibition, regarding the induction mechanism of imposex induced by organotins in gastropods are reviewed. Finally, a new hypothesis that states that retinoid X receptor (RXR) plays an important role in inducing the development of imposex, i.e., the differentiation and growth of male-type genital organs in female gastropods, is reviewed and discussed to clarify the entire mode of action of TBT or TPT in the development of imposex in gastropods.