Gigantism refers to a condition characterized by extreme physical size and stature. By definition, this originates during infancy, childhood or adolescence, while epiphyseal growth plates remain open. Although often used to specifically denote growth hormone excess, the term gigantism is also applied to a number of non-hormonally mediated overgrowth conditions in children. All forms of gigantism are extremely rare, and have in common a complex pathophysiologic origin and extraordinary clinical manifestations. Although many aspects of overgrowth disorders remain to be elucidated, insights into the molecular genetic basis of several forms of gigantism have contributed greatly to our understanding of specific mediators of growth and cellular function. Both growth hormone hypersecretion and overgrowth syndromes will be reviewed in this chapter.