This article summarizes the ontogenesis and genetics of the thyroid with regards to its possible congenital dysfunction and briefly refers to the roles of the mother-placenta-fetal unit, iodine effect, and organic and functional changes of the negative feedback mechanism, as well as maturity and illness, in some forms of congenital hypo- and hyperthyroidism. This article also describes the published literature and the authors' data on the clinical aspects of congenital hypothyroidism, on the alternating hypo- and hyperthyroidism in the neonatal period, and on neonatal hyperthyroidism.