In order to elicit a pharmacologic response following topical administration, antifungal agents must enter into and diffuse across the target biologic tissues, which have distinct architectures and compositions depending on their function. The rate and extent of transport will depend on the interplay between the drug's molecular properties and the characteristics of the biologic tissue. The drug may also interact with specific proteins or other membrane components. These interactions can prolong residence time and therapeutic effect; for example, azoles have an affinity for keratin (as do dermatophytes, their therapeutic target). Drug properties that increase permeability across a given membrane may render the molecule less effective at another biologic tissue; for example, the stratum corneum is a lipidic barrier, whereas the keratin-rich nail contains 10-fold less lipid and is perhaps best viewed as a hydrogel with very low lipid content. Consequently, both offer very different environments and drug delivery challenges compared with the oral and vaginal mucosae. In light of this, it is clear that formulation design and optimization are key steps in increasing the therapeutic efficacy of topical antifungal therapy.