The Wizard of Oz (WOz) technique is an experimental evaluation mechanism. It allows the observation of a user operating an apparently fully functioning system whose missing services are supplemented by a hidden wizard. In the absence of generalizable theories and models for the design and evaluation of multimodal systems, the WOz technique is an appropriate approach to the identification of sound design solutions. We show how the WOz technique can be extended to the study of multimodal interfaces and we introduce the Neimo platform as an illustration of our early experience in the development of such platforms.