This paper examines the rise of an epistemic genre, the Observationes, a new form of medical writing that emerged in Renaissance humanistic medicine. The Observationes (collections of case-histories) originated in the second half of the sixteenth century, grew rapidly over the course of the seventeenth, and had become a primary form of medical writing by the eighteenth century. The genre developed initially as a form of self-advertisement by court and town physicians, who stressed success in practice, over and above academic learning, as a core element of their professional identity. This unprecedented emphasis on practice as a source of knowledge remained a key feature of the Observationes in its subsequent development. As the genre evolved, the original emphasis on therapeutic success gave way to a new focus on the descriptive knowledge of disease through detailed observation. The authorial identity projected by the writers of Observationes was increasingly that of the learned and experienced observer, bent on comparing notes and sharing his cases with the fellow members of the res publica medica. This paper charts the development of the genre, examining how its growth contributed to the new epistemological value of observation in the age of the Scientific Revolution.