Polarized epithelia are fundamental to multicellular life. In animal epithelia, conserved cell-cell junctional complexes establish membrane diffusion barriers. In plants, the root endodermis resembles a polarized epithelium. The root endodermal cells form Casparian strips, belts of specialized cell wall material that generate an extracellular diffusion barrier. This breakthrough paper describes identification and characterization of the key regulators for Casparian strip organization. We will discuss the mechanisms underlying epitherial barrier formation in plants.