Leydig cells are the primary source of generating testosterone, and the differentiation of Leydig cells in testes is a signature event of the ontogenesis of males. Many studies elucidating the molecular changes and the mechanisms involved in the progressive differentiation of stem Leydig cells into adult Leydig cells have been appeared recently. Herein, the hormonal regulation on the ontogenesis of fetal and mature Leydig cells was reviewed in this paper.