This article introduces HephaisTK, a toolkit for rapid prototyping of multimodal interfaces. After briefly discussing the state of the art, the architecture traits of the toolkit are displayed, along with the major features of HephaisTK: agent-based architecture, ability to plug in easily new input recognizers, fusion engine and configuration by means of a SMUIML XML file. Finally, applications created with the HephaisTK toolkit are discussed. Categories and Subject Descriptors H.5.2 [Information Interfaces and Presentation]: User Interfaces