In the mid-twentieth century, physicians, engineers, mathematicians, philosophers, and others started to think about the ways in which a new technology might be applied to an old problem. These interdisciplinary teams designed computer programmes that aimed to automate one of the most fundamental tasks of medicine-diagnosis. This dissertation offers a history of computerising diagnosis through the prism of three such efforts: the Medical Data Screen, a computerised method of capturing the health of 'the total patient'; HEME, a computer programme created to aid in the diagnosis of haematologic diseases; and MYCIN, an early expert system developed to assist in the diagnosis and treatment of microbial infections. Drawing on these case studies, this project examines early efforts to computerise medical diagnosis and decision making. It explores how these efforts built upon, interacted with, and produced certain professional tensions, disease constructions, personal identities, cultural ideals, economic interests, and material practices. This dissertation supports a number of conclusions. First, it demonstrates that efforts to computerise medical diagnosis and decision making raised philosophical and moral questions that lie at the heart of medical practice. As physicians and engineers worked to computerise diagnosis, they were forced to reckon with fundamental epistemological and ontological questions-questions about the nature of clinical reasoning and the definitions of disease. In creating computerised diagnostic systems, early developers articulated new ideas about how physicians think and how disease categories are defined. Second, this dissertation argues that the introduction of computers into medical diagnosis was both the cause and the effect of new ways of collecting, storing, and using medical information. Third, this dissertation highlights the complexities inherent to medicine and medical diagnosis. Repeatedly early developers of computerised diagnostic systems ran up against unanticipated epistemological, social, and moral challenges.