Transdermal drug delivery systems (TDDS), commonly referred to as "patches," are medication dosage forms that give a regulated release of an amount of medication that is therapeutically efficacious over a patient's skin. This chapter goes into great length into the concept of skin, drug kinetics and penetration, TDDS formulation and evaluation, and its applications. Due to its distinct advantages, such as its prolonged therapeutic effect, avoidance of first-pass metabolism, and ease of therapy termination, TDDS has attracted intentional consideration for either local or systemic drug...