Most readers of Convergence will have some familiarity with the developing digital media forms that go under the name of augmented reality and mixed reality (MAR or separately, AR and MR). The widespread availability of smart phones in the last 10 years has redefined AR and MR that had previously been confined to the laboratory. Smart phones and tablets have become the platform for a variety of applications in which digital text, images, video, and audio are overlaid on the screen and appear to be present in the space around the user. In addition, the smart phone or tablet can typically determine the user's location in the world and orientation in his/her immediate environment. Along with the commercial uses for location-sensitive advertising, new forms of cultural expression (e.g. for art, design, and social media) are beginning to appear. Appropriately for this journal, these new forms can best be studied by a convergence of disciplines, including media studies, art history, literary theory, philosophy (particularly phenomenology), interaction design, sociology, anthropology, communication studies, human-computer interaction, and computer science. Many of these disciplines are represented in the contributions in this special issue that focuses on the ways in which AR and MR participate in cultural expression in today's heterogeneous media economy. Do AR and MR constitute a new medium? What are the specific qualities of the new medium that give rise to new forms of cultural expression? Are AR and MR two different media with different characteristic qualities and affordances? Over the past two decades, computer scientists have analyzed AR and MR as media forms from their own technical and operational perspectives (e.g. Milgram and Kashino, 1994). These questions are addressed from artistic and theoretical perspectives by the contributions to this special issue. The 'medium' question still underlies much of our discourse about the various digital technologies and their uses today, and the notion of medium has been naturalized today to such an extent that we may overlook its history in the 20th century. It is worth recalling that history in order to understand how the notion may limit our ability to appreciate the position that new forms such as AR and MR occupy in our media culture.