Nano- and microstructured materials differ from bulk materials because they are heterogeneous matrixes with large internal surface area. Interfaces inside nano- and microstructured composites can impact flux of probes through the matrix through unique interfacial chemistry and structure, interfacial binding and repulsion, and interfacial gradients. Gradients in properties are established at the interface between immiscible components; gradients generate forces which can drive flux. In microstructured composites, where a large fraction of molecules moving through the composite are exposed to interfaces and their associated chemistry, structure, and gradients, effects unimportant in homogeneous materials should be considered. First, the relationship between structure and flux can be evaluated by varying the ratio of internal surface area to internal volume of the composite. Structure-flux relationships provide design constraints for forming composite materials. Second, the structure of the interface itself impacts flux. Third, structure-flux relationships will fail as characteristic dimensions of the nanostructure approach molecular dimensions. Examples from the literature are used to illustrate these points.