Recent progress in an understanding of the role of thyroid hormone (TH) on fish development was reviewed with particular reference to fish farming. TH is a small liposoluble hormone produced in thyroid follicles and has two bioactive forms, tetraiodothyronine (thyroxine, T₄) and triiodothyronine (T₃). Mother fish deposits a considerable amount of TH in developing oocytes. The amount of TH and ratio of T₃ to T₄ in eggs vary among species and stocks of the same species. The role of TH in eggs is not yet clear, though enhanced survival and growth of embryos and larvae by the enrichment of the eggs with TH have been reported in some cases. On the other hand, the role of TH that promotes larva-juvenile transition or metamorphosis has been established in teleost fish. The level of TH elevates at the onset of metamorphosis, and the treatment with TH can induce precocious metamorphosis. Considering the TH role in fish development, the disorder of its proper function could lead to deformity in juveniles, which is frequently observed in hatchery. In addition, many environmental chemicals have potential disrupting TH function in developing embryos and juveniles of fish.