This article proposes an evaluation framework to benchmark the performance of multimodal fusion engines. The paper first introduces different concepts and techniques associated with multimodal fusion engines and further surveys recent implementations. It then discusses the importance of evaluation as a mean to assess fusion engines, not only from the user perspective, but also at a performance level. The article further proposes a benchmark and a formalism to build testbeds for assessing multimodal fusion engines. In its last section, our current fusion engine and the associated system HephaisTK are evaluated thanks to the evaluation framework proposed in this article. The article concludes with a discussion on the proposed quantitative evaluation, suggestions to build useful testbeds, and proposes some future improvements. Categories and Subject Descriptors H.5.2 [Information Interfaces and Presentation]: User Interfaces