Precocious puberty represents a unique diagnostic problem in which imaging plays an important role. Development of secondary sex characteristics may result from inappropriate activation of the hypothalamic-pituitary axis with release of gonadotropin, or from gonadotropin-independent secretion of sex steroids by the adrenal glands or gonads. A variety of lesions can manifest with precocious puberty, including various central nervous system (CNS) lesions, adrenal lesions, and sex cord-stromal tumors of the testis or ovary. CNS lesions causing precocious puberty are much more common in boys than in girls and are well evaluated with brain magnetic resonance imaging. Neoplastic (hypothalamic-chiasmatic astrocytoma, suprasellar germinoma) and nonneoplastic (hypothalamic hamartoma, hydrocephalus, trauma, empty sella, infection, congenital midline anomalies) conditions may affect the function of the hypothalamic-pituitary axis. The adrenal cortex may produce sex hormones. Some adrenal cortical neoplasms (ACNs) in patients under 5 years of age are related to a mutation of the tumor suppressor gene p53 and represent a disease that is distinct from ACNs in older children and adults. Adrenal cortical hyperplasia secondary to an enzymatic defect in steroid biosynthesis causes virilization and salt wasting, which usually manifest in the neonatal period; however, milder forms of the disease may manifest in childhood. Female precocious puberty may be caused by an autonomously functioning ovarian cyst or a juvenile granulosa cell tumor of the ovary. Male precocious puberty may be caused by a sex steroid-producing Leydig or Sertoli cell tumor of the testis. Ultrasonography is the primary modality for evaluating the sex organs and may also be used to evaluate for adrenal abnormalities.