Electrically conductive (EC) coatings on optically transparent windows can be used to protect internal system's components against electromagnetic interference effects. The purpose of this paper is to derive simple but correct formulas for assessing the far-field shielding effectiveness of a thin metallic film or multilayer stack deposited on top of a dielectric slab. Using conventional transmission-line theory in conjunction with proper expressions for the transmittance of a perfect dielectric, the author demonstrates that, for coatings much thinner than the skin depth, the following holds. CO In a half-wave geometry, the shielding effectiveness (in decibel) is a function of the sheet resistance only: (SEλ/2 = 20log(1+188.5/Rs) if Rs is in ohm per square. And (b), in a quarter-wave geometry, the shielding effectiveness is best approximated by means of a semiempirical expression, (SE)λ/4 = 201og(1+Er)/(2√Er) + 188.5/(√ErRs), where Er refers to the dielectric constant of the substrate. These formulas provide upper and lower limits for the effective shielding performance of an EC-coated window, depending upon the phase thickness; they accurately predict measured microwave attenuations for a variety of visible- and infrared-light transmitting domes as well as for standard conductive-glass windows.