There has been a long controversy over the factors determining caste development of individuals in social insects. It has been known that parental phenotypes influence the caste fate of offspring in termites. Recently, we demonstrated that parental phenotypes influence the social status of the offspring not through genetic inheritance but through genomic imprinting. We conducted extensive field survey and genetic analysis of the termite Reticulitermes speratus and concluded that the caste system of this species cannot be explained by genetic caste determination models. Alternatively, we documented a genomic imprinting caste determination system in termites, in which queen- and king-specific epigenetic marks antagonistically influence the sexual development of offspring and thus determine their caste fate. Our genomic imprinting model accounts for all known empirical data on caste differentiation in termites and explains the evolutionary processes underlying diverse reproductive systems. According to the genomic imprinting model, the worker caste is seen as a ‘neuter’ caste whose sexual development is suppressed due to counterbalanced maternal and paternal imprinting. We further propose a novel hypothesis that genomic imprinting played an essential role during the transition from subsocial to eusocial.