The term 'micropenis' is applied if the measured penile length is more than 2.5 standard deviations below the mean for the age; in neonates born at term this means a penile length of less than 2 cm. Micropenis is the consequence of a defect which develops after the 14th week of pregnancy. The cause may be hormonal (hypothalamo-hypophysial (hypogonadotropic hypogonadism), testicular (hypergonadotropic hypogonadism) or end organ resistance), but also iatrogenic (medication during the pregnancy). In addition, micropenis occurs as a part of a number of syndromes. The diagnostic investigation comprises history-taking, physical examination, specific hormonal testing and, if considered necessary, chromosomal and imaging examinations. In the treatment of micropenis, a trial treatment with testosterone plays an important part.