Some types of polyphenism are known to be under the control of juvenile hormone (JH). These polyphenisms include morphological changes, such as weaponry and body coloration; changes in internal organs, such as the gonads; and behavioral changes. As a notable example of polyphenism involving many changes, this chapter summarizes recent advances in research on the regulatory mechanisms of caste differentiation in termites by JH and other factors and signal transduction pathways. In addition to caste differentiation in hymenopteran social insects, the role of JH in other plastic phenomena, such as male weapon polyphenismWeapon polyphenism in beetles, changes in reproductive modes in aphids, and phase polyphenism in locusts, will be presented.