This document provides an overview of transdermal drug delivery systems (TDDS). It discusses the key components of TDDS including polymers, drugs, permeation enhancers, pressure sensitive adhesives, backing laminates and release liners. It also summarizes the different types of transdermal patches including single-layer drug-in-adhesive, multi-layer drug-in-adhesive, reservoir, and matrix patches. Finally, it briefly discusses the formulation approaches used in developing TDDS and some factors that can affect skin permeation.