The primary insect steroid hormone ecdysone induces molting and metamorphosis through its interaction with intracellular nuclear receptors. Although it has long been assumed that steroid hormones can enter their target cells by simple diffusion, we recently demonstrated that a membrane transporter named Ecdysone Importer (EcI) is required for cellular incorporation of ecdysone in the fruit fly Drosophila melanogaster. To further explore the physiological significance of steroid hormone transporters, we are continuing our study in many different directions. I will present our most recent results and discuss the importance of transporter-mediated cellular incorporation of steroid hormones.