Disorders of pubertal timing are common and challenging problems for pediatric endocrinologists. Early or late puberty can have immediate effects on a child's psychosocial well-being and may have long-term effects on adult stature. Much is known about the regulation of the hypothalamic-pituitary-gonadal axis, but the triggers of pubertal onset in the general population remain elusive. This article reviews recent data suggesting a possible shift in the age of pubertal onset; current knowledge regarding factors that regulate the onset of puberty; and the etiologies, diagnosis, and treatment of precocious and delayed puberty.