Puberty is a developmental stage during which endocrinological, physical and psycho-social changes proceed. During prepubertal period, hypothalamic-pituitary-gonadal axis is maintained in an inactive state by negative feedback regulation or inhibitory function of central nervous system. The onset of puberty and sexual maturation are gained by the alternation of these inhibitory system. In recent years, highly sensitive techniques for evaluating the hormones of hypothalamic-pituitary-gonadal system have been developed. And many explorations of the hormonal mechanisms of puberty have been achieved. In this chapter, hormonal changes in puberty is reviewed, and mechanism of puberty in men is discussed.