Systemic crop protection products are commonly sprayed onto foliage, whereupon the active substances must penetrate into the leaves in order to become biologically active. Penetration of the plant cuticle is the rate-limiting step. The diffusion of organic non-electrolytes within cuticles is a purely physical process that can be described and analysed in the same way as is done for diffusion in synthetic polymer membranes. Solute mobilities in cuticles vary considerably between plant species. For a given species they decrease with increasing solute size, and this size selectivity holds for all of the plant species investigated so far. Wax extraction from leaf cuticles increases the mobility of solutes tremendously, but size selectivity is not affected. Furthermore, diffusion within plant cuticles is extremely temperature dependent. An analogous increase in solute mobility can be achieved by using accelerators, which enhance the fluidity of the polymer matrix and of the waxes. The effects of temperature and plasticizers on the diffusion of non-electrolytes in wax and the cutin matrix have been used to characterize the nature of the lipophilic pathway. The 'free volume' theory can be used to explain the influence of the size and shape of the solute, and its dependence on temperature. The physico-chemical nature of the diffusion pathway has been shown, by thermodynamic analysis, to be identical for a wide range of solute lipophilicities. This approach also explains the mode of action and the intrinsic activity of plasticizers.