Establishing diagnoses is a crucial aspect of medical practice. However, this process has received comparatively little logical and pedagogical attention when compared to statistical methods for evaluating evidence. This article investigates the logic of medical diagnosis in order to fill this void. It is organized in three parts: the first attempts to explain why more attention ought to be paid to diagnosis, at least as much as to evidence; the second calls attention to the method of diagnosis by abductive reasoning developed in the 19th century by Charles Sanders Peirce (1839-1914); and the third demonstrates the use and pervasiveness of abduction by any other name in clinical diagnosis. We examine six diagnostic strategies in common use that contain most, if not all, of Peirce's structure of inquiry in science.