The two complementary views of human-computer interaction, i.e. perception and control, have been described in isolation in the two previous chapters. Their combination --- with respect to multimodal systems --- is the topic of this chapter. We will start with an overview on existing architectures and interaction models. In the second section, these theoretical considerations will be directed towards practical issues of input/output coupling, followed by a section on synchronization. In the last two sections, aspects of Virtual Reality systems and an analysis of interaction will be explained.