Drug penetration and permeation through the skin are greatly influenced by the
structural properties of the skin and the physicochemical properties of the drug.
As such, penetration enhancement techniques largely focus on manipulating
these two key factors. A great deal of research has focused on the stratum
corneum, the primary skin barrier. In this chapter, we describe the structural
properties of human skin, its functions, and the basic principles of drug penetra-
tion. The lipid composition and structural organisation of the stratum corneum,
as well as the pathways of drug permeation, are highlighted. This chapter
should provide a basic understanding of these topics and prepare the reader for
advanced discussions in the specialist chapters that follow.