What counts as scientific writing has undergone massive changes over the centuries. Medical writing is a good representative of the register of scientific English, as it combines both theoretical concerns and practical applications. Ideas of health and sickness have been communicated in English written texts for over a thousand years from the Middle Ages to the present, with different traditions and layers of writing reflecting literacy developments and changing thought-styles. This chapter approaches the topic from the perspective of registers and genres, considering how texts are shaped by their functions and communicative purposes and various audiences. Some genres run throughout the history of English: remedy books were already extant in the Old English period. Another core genre, the case study, mirrors wider scientific developments in response to changes in styles of thinking: medieval scholasticism is gradually replaced by a growing interest in increasingly systematic empirical observation. The establishment of learned societies from the seventeenth century onwards gives rise to new genres like the experimental report, and concomitant disciplinary advances and technological developments in the following centuries gradually pave the way for modern evidence-based medicine. Today medical advances are communicated in digital publications to a worldwide readership.