Skin, once thought to be an impenetrable barrier, is an extremely active organ capable of interacting with its environment. Advancements in science combined with the need for diverse drug delivery modalities have introduced a variety of transdermal and intradermal products for existing drugs at a fraction of the cost of new drug development. Interest in drug delivery systems is growing as the delivery of many drugs continues to be a problem due to safety, stability, and compliance issues. This is especially true for the increased number of biologic medicines, which include peptides, proteins, and nucleic acids. Transdermal delivery is an important part of such systems, and the use of physical enhancement methods to penetrate the skin barrier is where the field is headed. This book reviews the strategies available for the delivery of compounds into and through the skin. It also focuses on technological developments in the domain of nanotechnology that can be exploited to enhance the transdermal delivery of drugs.