Based on a number of revolutionary ideas in the early seventies the development of transdermal delivery systems (TDS) for systemic therapy has been receiving considerable attention, both in academia and in the pharmaceutical industry. About ten years after a number of products were successfully put on the market, it has become clear that transdermal delivery of drugs not only exhibits appealing therapeutic prospects but may also provide a viable economic basis for future activities of the pharmaceutical industry. The paper will briefly stress some of the fundamentals of transdermal delivery with respect to the skin as absorption site. The main focus will be on transdermal delivery systems, covering design of transdermal systems, manufacturing of transdermal systems, polymers and adhesives, in vitro and in vivo testing of transdermal systems, principles of delivery control, and modelling of transdermal delivery. The final section will describe the fundamental strategies for enhancing drug permeation through the skin by alteration of barrier properties, approaches for thermodynamic activity increase of drug, and the iontophoretic transdermal delivery of drugs.