Therapeutic systems were born of the recognition that the actions and safety of hormones depend not solely on chemical structure but also on their precisely controlled endogenous delivery. In contrast, conventional drug dosage forms exert no control, permitting the familiar peak-and-trough concentration cycle, often resulting in side effects or suboptimal therapy or both. Therapeutic systems were conceived to optimize treatment by delivering drug according to a therapeutically rational program that would select desired actions from a spectrum of concentration-dependent actions. The systems' design concept incorporated a reservoir isolating drug from the body, a rate controller, an energy source, and a platform. Because the program is the systems' raison d'etre, we considered them not devices but rate- and duration-controlled drug products requiring Food and Drug Administration approval. Our decision to create these products, rather than licensing the technology to others, reflected our own commitment to advancing therapy, not simply creating "hardware" for delivering drugs. Milestones in systems development were discussed, including the uncovering of therapeutic values and optimal delivery patterns along the way. Finally, future challenges for therapeutic systems development in the biotherapeutic era were briefly sketched.