Counterfeiting of products and important documents is at an all-time high and is costing the world economy hundreds of billions of dollars yearly as well as posing significant safety and health hazards through the production of uncertified goods, e. g., pharmaceutical products. To limit these effects, interference-based optical security devices offering an angular color shift are still widely in use. Unfortunately, commercial iridescent materials are now readily available and represent a potential source of counterfeiting. In this short review, we first describe the basic principles behind passive interference security image structures (ISIS) and the qualities which have resulted in their integration into most important documents. Various features which have been added to ISIS in order to make them harder to duplicate yet simpler to authenticate are also presented (metamerism, magnetic materials, diffraction, etc.). We then address the implementation of active materials, mainly electrochromic WO3 as a means of generating two-level authentication devices. Finally, we discuss some general considerations to keep in mind when developing features for security applications. (C) 2013 Elsevier B.V. All rights reserved.