Growth hormone (GH) secretion from anterior pituitary is regulated by the hypothalamus and the mediators of GH actions. Major regulatory factors include GH releasing hormone (GHRH), somatostatin (SRIF), GH releasing peptide (ghrerin) and insulin-like growth factor (IGF-I). The principal physiological regulation mechanisms of GH secretion are neural endogenous rhythm, sleep, stress, exercise, and nutritional and metabolic signals. GH deficiency results from various hereditary or acquired causes, which may be isolated or combined with other pituitary hormone deficiencies. GH deficiency can be treated with recombinant human GH, which results in accelerating growth in children and normalization of intermediary metabolism in adults. GH hypersecretion mostly results from a pituitary tumor and causes acromegaly or gigantism. Hypersecretion of GH can be treated by transshenoidal surgery. Medical treatment with octreotide and analogs is also effective to reduce GH secretion in combination with or without the surgery.