Since puberty is a long ongoing developmental process with significant individual and population differences in timing, the definition of delayed puberty for a given individual is based on arbitrary criteria given by epidemiological data. Delayed or late puberty is discussed when there is no breast development at an age that is 2 to 2.5 SD later than the mean age at which this event occurs i.e. 13 years. The differential diagnosis between delayed puberty and absence of puberty i.e. hypothalamo-pituitary defects or ovarian failure is still puzzling: there is no gold-standard test with a sufficiently high degree of sensitivity/specificity that could allow one to differentiate: - variability of pubertal development - hypogonadotropic hypogonadism (genetically or functional). Late puberty can affect psychosocial wellbeing of adolescent girls: beside anxiety and depression symptoms, decreased self-esteem, concerns regarding development of sexual identity and fertility are frequently reported. Psychological support should be systematically discussed. First line evaluation includes: - family history - physical examination (Tanner stage, growth rate) - hormonal analyses (plasma level of FSH, LH, PRL, IGF-1) - imaging (bone age, pelvic US). Second line investigation associates: - GnRH stimulating test - olfactory function test - brain MRI - genetic and molecular genetic analyses. Diagnosis of a girl with delayed/absence of puberty includes: 1. Constitutional delayed puberty (and growth) (CDPG): about 25%. No test can reliably distinguish CDPG from isolated hypogonadotropic hypogonadism (IHH). IHH is diagnosed if puberty is not begun by the age of 18 years. 2. Hypogonadotropic hypogonadism: about 45%. It can be: - acquired: post-tumoral, post-radiotherapy - functional: stress, anorexia, sport - congenital: isolated GnRH deficiency. 3. Hypergonadotropic hypogonadism: 30%. It can be: - acquired: post-chemotherapy, … - congenital: XO (Turner), XX (premature ovarian insufficiency), or XY (DSD). Treatment should be discussed, according to the cause of delayed/absence of puberty, in agreement with the adolescent personal choice.