Conductive coatings have been applied to plastic substrates to protect electronic devices against electromagnetic interference. A model, based on the transmission line and plane wave theory, is developed to analyze the shielding effectiveness of multilayer metallic thin films. Analyses show that among absorption, reflection, and re-reflection in electromagnetic wave transmission, reflection is dominant, whereas absorption is negligible because of small film thickness. A key indicator of reflection is the thin film's intrinsic impedance characterized by the ratio of conductivity over permeability. Better shielding can be achieved by having the impedance ratio of the adjacent layers higher than 1; i.e., by placing the thin film of higher impedance as the inner layer to the substrate. Without the correct sequence of placement, more layers do not necessarily lead to better shielding. All analytical results are validated by experiments on PC and PP substrates with plasma surface treatment and physical vapor deposition. (C) 2008 Wiley Periodicals, Inc.