The term “medium” has a long and complicated history. In its most general sense, it originally meant an intermediate agency, instrument, or substance. During the 19th century, it acquired two further common meanings, first as the raw material or mode of expression distinguishing a particular artistic practice, and then, in the sense now prevalent, as a channel of mass communication. For much of its history, literature’s primary medium (or “channel”) has been the printed book, which remains an object of theoretical as well as historical enquiry, often in a comparative context. Since around 1850, the proliferation of technical media—from telegraph and telephone through film, radio, and television to the internet and the mobile phone—has piled comparative context upon comparative context for the study of literature to take into account. From its origins in the 1920s, media theory has tended to reverse engineer an understanding of what a “medium” is and does from its description and analysis of specific material technologies in operation. Its original focus, with film, radio, and television in mind, was on the medium as technical and ideological instrument. However, as material technologies have become ever widespread, sophisticated, and diverse, so their function has begun to resemble that of an intervening agency or substance, rather than that of an instrument.