In this chapter, we describe the histophysiology of human Leydig cells, their cytological characteristics, their differentiation processes, and the physiopathological processes occurring at various times throughout life. We first focus on the normal development of fetal Leydig cells as well as the pathologies of fetal Leydig cells that can affect numbers or hyperplasic processes (e.g., hypogonadotropic hypogonadism, cryptorchidism, congenital Leydig cell hyperplasia secondary to diabetes, and isoimmunization). Next, we explain the changes occurring at puberty with the onset and differentiation of adult Leydig cells and the pathophysiology of delayed puberty. We then describe the histophysiology of adult Leydig cells and the most frequent pathologies (e.g., hypogonadotropic hypogonadism, testicular dysgenesia, mild androgen insensitivity syndrome, 5-α-reductase defect, and Klinefelter syndrome). Finally, we discuss the morphological changes of these cells in the elderly
