Introduction.- What Are Multimodal Systems? Why Do They Need Evaluation? - Theoretical Background.- What to Evaluate? - A Taxonomy of Quality Aspects of Multimodal Interfaces.- How to Evaluate? - Development of the MMQQ (MultiModal Quality Questionnaire).- Is the Whole the Sum of its Parts? - Predicting the Quality of Multimodal Systems Based on Judgments of Single Modalities.- What Determines Modality Selection Strategies? - Identifying Factors Influencing...