Juvenile hormone is a key hormone in the control of insect development and reproduction. This chapter provides a brief overview of its discovery and subsequent identification as a sesquiterpene, the first example of this type of lipid serving as a circulating hormone. We then describe its biosynthesis and the relationship of its circulating titer to the diverse life history strategies that resulted in the great array of insect species that are evident today. We then turn to the cellular and molecular actions of juvenile hormone. Its receptor is a basic-helix-loop-helix transcription factor. The identification of the juvenile hormone receptor has led to a better understanding of the relationship of this hormone to the molting hormone, ecdysone, and an understanding of its ligand-binding properties has opened the door to the long-standing promise of using juvenile hormone-like molecules for selective insecticides. Some aspects of juvenile hormone action, though, remain poorly understood. The morphostatic actions of juvenile hormone likely involve interactions with both local and circulating morphogen systems, and details of these interactions are yet to be revealed.