My doctoral research focuses on the usability and usage of new computer technology such as interactive systems that support the combination different input media such as voice, gesture and video. I have contributed to the study of these systems in four complementary ways: the MSM framework helps classifying and reasoning about current and future "interactionally-rich" systems; I have also used formal methods to specify multimodal interaction; I have designed and developed the NEIMO multimodal Wizard of Oz platform, a tool for observing users using multimodal interaction and assessing usability issues; I am now working on computer-mediated communication and collaboration through audio and video.