This chapter discusses the key events occurring at different stages of development in the normal human male and how this impacts the reproductive function in adulthood. Development of the normal male reproductive system can be divided into five periods: fetal, neonatal, infancy/childhood, puberty, adulthood. Normal hormonal functioning of the fetal testis is essential for masculinization and for development of a male reproductive system. In neonatal male and female babies there is activation of the hypothalamic-pituitary axis and consequent increase in circulating levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). During the neonatal period, Sertoli cells continue to proliferate and this is arguably the most important period in life in terms of the magnitude of increase in Sertoli cell numbers. Sertoli cells start expressing androgen receptor (AR) at the onset of puberty and this is considered one sign of maturation of these cells, as they terminally differentiate and cease proliferating.