Drug development, either for topical or transdermal products, represents an added layer of both complexity and opportunity for the drug developer and clinician. Whether a topical or transdermal formulation, the bioavailability of any topically applied product is the result of the complex interaction between the drug substance, formulation-dosage form and skin factors that impact the barrier function of the skin. Moving from the general to specific routes of delivery, topical drug products represent a broad variety of dosage forms and presentations including creams, ointments, lotions, gels, sprays, powders and mousses. Understandably, transdermal drug delivery offers several important advantages over more traditional dosage forms. For transdermal systems, the combination of adhesives and the administration of the drug itself under the occlusive nature of transdermal drug delivery systems for a prolonged duration can raise the potential for local reactions. Dermatologic conditions range in surface area involvement from almost total involvement to lesional in nature.