This thesis conceptualises, investigates, and reflects on the moving image design space in Human-Computer Interaction (HCI). Motivated by the increasing number of videos, films, and animations produced in the field, the thesis recognizes moving image making as a designerly way of inquiry across research and practice, and argues for the importance of moving image as a research topic in interaction design. The first contribution of this thesis is the conceptualization of the moving image design space. The growing body of moving images, varying in forms and purposes, can be held together to establish a foundation of knowledge that informs and generates new research and practice. We identify four collections of existing works and their different roles, namely moving image as design technique , design element , design exhibit , and design promotion . The second contribution is the manifestation of moving image making through concrete design studies. These exemplars empirically demonstrate how they investigate, enrich, and challenge the four established collections, and ultimately expand the moving image design space. These contributions not only provide new knowledge on moving images for better understanding their various roles in interaction design and making works that respond to emerging design opportunities, but also foreground the discussion on the mediation aspect of moving image in HCI.