This article conceptualises diagnosis as ongoing practical judgement in medical care. Based on pragmatist and phenomenological considerations of tools in use, it uses a comparative approach to analyse similarities and differences in the use of diagnostic technologies. In the first part of the paper, a historical perspective on the innovation of the stethoscope is used to highlight the transformations in diagnostic practices occasioned by novel diagnostic instruments. In the second part of the paper, ethnographic accounts of contemporary anaesthesia are presented in order to sketch out the manifold variations of using diagnostic instruments in daily practice. Both cases are analysed on a micro-analytical level, emphasising the interrelations of bodies, tools and knowledge in concrete situations. The analysis shows how diagnostic instruments become embodied in the perceptual habits of physicians and how diagnosing becomes an ongoing activity in the course of managing an illness trajectory.