Two important issues in multimodal system design, is the selection and mix of input and output modalities and the exploitation of the synergies between the modalities in order to maximize the usability of the system. In this talk, we propose two new objective metrics, relative modality efficiency and multimodal synergy, that can provide valuable information and identify usability problems during the evaluation of multimodal systems. Relative modality efficiency (when compared with modality usage) can identify suboptimal use of modalities due to poor interface design or information asymmetries. Multimodal synergy measures the added value from efficiently combining multiple input modalities, and can be used as a single measure of the quality of modality fusion and fission in a multimodal system. The proposed metrics are used to evaluate a multimodal system that combines pen and speech input, and are compared with traditional evaluation metrics. The results provide much insight into multimodal interface usability issues, and demonstrate how multimodal systems should adapt to maximize modalities synergy resulting in efficient, natural, and intelligent multimodal interfaces.