While the role of medicine in early modern theatre has been studied extensively in recent years, little attention has been paid to the intersections of theatre and medicine in modern drama. The second half of the nineteenth century and the first quarter of the twentieth witnessed innovations in both fields, as a self-styled "modern" theatre asserted itself against the conventions of an earlier period, and medical science claimed new authority as part of the emerging network of modern disciplines. One impediment to recognizing the role of the medical in modern drama has been the prominence of realism within this tradition. Because of realism's investment in the ideology of the visible, the interior landscape of disease is consigned to one of its many offstage realms, while the theatrical use of technological devices by which this realm might be disclosed to the audience – image and sound projection, X-ray imaging, and so on – are precluded from strictly illusionistic theatre. But realist (and naturalist) theatre is more deeply implicated in medical ways of seeing than its representational limitations suggest. Because of the tensions within realism between illusionism and actuality, realism is uniquely poised to reveal the somatic body in its normal and pathological states and in its physiological interaction with its environment. The many doctors who people modern drama are threshold figures who negotiate the social visibility of illness, demarcate the line between normal and pathological, and often represent theories and disciplinary ideologies that construct the human in medical terms. By modelling various forms of scientific and medical observation, theatrical doctors also direct attention to the complex symptomology of modern drama, in which the boundary between the body's surface and interior becomes the site of spectatorial interpretive operations.