This chapter reviews a variety of ecdysteroid functions in the insect life cycle, with the goal of understanding them in a comprehensive manner at the molecular level. Ecdysteroids can affect multiple tissues simultaneously and directly change their gene expression profiles. The expression profile of Halloween genes during embryonic development suggests that epidermal cells are the major source of embryonic ecdysteroids. The prothoracic gland is the primary site of ecdysteroid production during post-embryonic development. Ecdysteroids thus regulate organ growth in both direct and indirect ways, and thereby play a central role in shaping adult body structure in insects. Downstream effector genes involved in ecdysteroid-mediated neuronal remodeling are being revealed using the Drosophila melanogaster model system. Functions of ecdysteroids in adult female reproduction have mostly been studied in dipteran species, particularly in the yellow fever mosquito Aedes aegypti and D. melanogaster. In mosquitoes, males even transfer ecdysteroids to females upon copulation and change their reproductive behavior and physiology.