The efficacy of actives applied by the topical route is limited not just by factors relating to the active ingredient(s) but also formulation design and biological factors such as skin permeability. Topical formulations have to be designed to ensure that a sufficient dose of the active molecule reaches its target site, be it the skin surface, the skin itself or the underlying cutaneous vasculature. In order to achieve this, the active substance has to overcome the natural barrier function of the skin. Several strategies have been studied to enhance skin penetration and so improve cutaneous bioavailability.