This thesis is about the process and practice of diagnosis and the implications of new diagnostic technologies in low resource settings. As a setting and a disease which has seen significant investment in diagnostics, Lassa fever in Sierra Leone has been selected as a case study to examine these themes. In this thesis, 'new diagnostic technologies' refers to laboratory-based diagnostics which are fast, reliable, accurate and can be used in low income settings. The starting point of this thesis is a narrative surrounding such technologies which suggests that they will revolutionise low income healthcare settings by allowing accurate scientific diagnosis in places where it was not possible before. Various perspectives on diagnosis are examined and some limitations are identified in relation to their accounts of diagnostic process, context, practice and technology. To explore the case, aspects of science and technology studies, the sociology of scientific knowledge and medical anthropology are combined. A multi-sited ethnography of Lassa fever diagnosis was conducted in three settings: a rural village, a laboratory and the wards of a hospital. Documents were reviewed and interviews conducted with key actors and ex-Lassa fever patients. Analysis focused on framings (partial and subjective interpretations), narratives (persuasive storylines which make use of particular framings) and practice in relation to Lassa fever and the development of technology for its diagnosis. Assumptions about the disease, diagnostics and the process of diagnosis are identified and the conclusion considers how they compare with practice in each setting. This thesis argues that diagnosis is a complex negotiated process and that new diagnostics represent only one aspect of that process. Thus, they are not a ‘silver bullet' to transform low resource healthcare contexts. In particular, ‘improved' diagnostics do not always have the expected impacts, sometimes even introducing complexity and uncertainty. In challenging narratives about diagnostics, this thesis provides an alternative, practice-based, approach to thinking about diagnostics and innovations in health systems; this approach acknowledges the importance, and complexity, of the diverse contexts which shape innovations and technology use.