Publisher Summary This chapter discusses the juvenile hormones (JHs) chemistry, biological action. JHs together with ecdysteroids orchestrate the expression of larval-specific genes each time the insect molts to a new stage. These morphogenetic effects include determination of an immature body form and internal organs, hardness and color of the cuticle, and accompanying physiology and behaviors. At the conclusion of larval development, JH levels drop at critical times, allowing ecdysteroids to program expression of pupal and adult characteristics. JHs return in the adult stage, in which they have gonadotropic functions in connection with reproduction. In addition to their morphogenetic and gonadotropic actions, JHs are involved in dormancy and various types of polyphenisms, including caste determination in social insects. The JHs are lipophilic sesquiterpenoid derivatives of farnesoic acid. Their chemical nature came into focus in the early 1960s, beginning with observations that farnesoid components of beetle excreta had JH-like activity in bioassays. The presence of JHs in the blood promotes expression of juvenile characters, chief among these being an immature body form or morphology. For insects such as grasshoppers, which undergo incomplete metamorphosis, the effects are not so visible outwardly. The early stages look like miniature adults except for the absence of wings, but they also lack functional reproductive organs.