The studies of basic phenomena of silkworm growth and development, and their regulation mechanism are of instructive significance to establish the rational rearing method, to produce high quantity and high quality cocoons and silks. This chapter firstly discussed the metamorphosis phenomena of silkworm and its physiological and biochemical changes. Then the chapter mainly discussed the regulation of hormones in growth and development of the silkworm, especially for molting hormone and juvenile hormone in their secretory organs, chemical structure, biosynthesis and inactivity. Further the chapter displayed the action mechanism of molting hormone and juvenile hormone during silkworm metamorphosis. Lastly the chapter discussed the control of corpus allatum and prothoracic gland by the brain and the brain-corpus allatum-prothoracic gland system and metamorphosis.