Puberty is the phenomenon that conducts once to reproductive maturation. Delayed puberty (DP) is defined by the absence of testicular development in boys beyond 14 years old (or a testicular volume lower than 4 ml) and by the absence of breast development in girls beyond 13 years old. DP occurs in approximatively 3% of cases. Most cases are functional DP, with a large amount of constitutional delay of puberty. Others etiologies are hypogonadotrophic hypogonadism like Kallmann syndrome, or hypergonadotrophic hypogonadism. Turner syndrome is a diagnostic one should not forget by its frequency. Treatment is hormonal replacement therapy and of the etiology. During the last decade, many genes have been identified and elucidated the etiological diagnosis of some hypogonadotrophic hypogonadism syndrome. Further studies are required in collaboration with molecular biologists to better understand the mechanism of hypothalamic pituitary gonadal axis abnormalities and of the neuroendocrine physiology of the onset of puberty.