Merja Kytö is Professor of English Language at Uppsala University. In this article, she provides a detailed accounting of the role of register in research on the historical development of language. Her substantial body of work has focused on both the historical development of specific registers, as well as how historical change has been mediated by register. Her research has encompassed a range of time periods (from Early Modern English to the 19th century) and registers (for example, depositions, Salem witchcraft records, and dialogues). Her many edited collections have brought historical linguists together into comprehensive and rigorous volumes, including the Cambridge Handbook of English Historical Linguistics (Kytö & Pahta 2016, Cambridge University Press), English in Transition: Corpus-Based Studies in Linguistic Variation and Genre Styles (Rissanen, Kytö, & Heikkonen 1997, De Gruyter), and Developments in English: Expanding Electronic Evidence (Taavitsainen, Kytö, Claridge, & Smith 2014, Cambridge University Press). She has been a key contributor to the development of principled historical corpora, such as the Helsinki Corpus of English Texts project, which represents a range of registers from Old and Middle English to Early Modern English. Merja Kytö has long been a leader in demonstrating how systematic attention to register can result in rich profiles of historical development, and in addressing the inherent challenges involved in utilizing historical documents for linguistic research.