A theory of charge, fluid-mass, and solute (including macromolecular) transport through porous media is applied to describe transport phenomena across the external layer of mammalian skin. Linear relationships are derived between transport fluxes and applied fields. These relationships introduce six effective transdermal transport coefficients. Formulas for each of these coefficients are provided. The practical relevance of these parameters is emphasized in the specific context of transdermal drug delivery. By employing typical physiological values for the various geometrical and physicochemical parameters that appear in the formulas for the transdermal transport coefficients, predictions are made for transport rates of charge, fluid mass, and solute species across a uniform-thickness skin sample contained within a diffusion-cell apparatus. These results are used to explore transdermal phenomena involving forced convection, current flow, electroosmosis, iontophoresis, and molecular diffusion (including convective dispersion). Comparisons with existing transdermal drug delivery data are made. On the basis of these comparisons, the theory suggests that transdermal transport in the presence of an electrical field may occur through corneocytes of the stratum corneum. The theory confirms the importance of a shunt route for small ion transport, as well as an intercellular route of transport for passive diffusion of noncharged substances. These latter conclusions, also based on comparisons with experimental data, are consistent with previous statements in the literature. A new form of solute transport enhancement, termed transdermal convective dispersion, is included in the theory, and methods for its measurement are described. Generalizations and future applications of the theory are discussed.