Franz Leydig’s groundbreaking identification of adult Leydig cells in 1850 prompted a widespread investigation into the roles and regulation of these specialized cells within the testes (Leydig, Z Wiss Zool 2, 1850). In the early 1900s, Whitehead conducted pioneering research on fetal Leydig cells in pig embryos, establishing the concept of a distinct Leydig cell population present during fetal development [2, 3]. Subsequently, it became widely acknowledged that most mammalian testes harbor at least two populations of Leydig cells (fetal and adult), each active at different stages of development. Despite variations in their morphology and biochemical properties, both fetal and adult Leydig cells share a primary function: the synthesis of androgens. Fetal Leydig cells have been extensively studied across a range of mammalian species, including humans, primates, pigs, rabbits, guinea pigs, hamsters, rats, and mice. This chapter specifically explores the morphological characteristics, formation, specification, and functions of Fetal Leydig cells in rats and mice, which serve as primary models for physiological and genetic experiments in this field.