This Element explores interdisciplinarity in academic writing. It describes the ways in which disciplines interact when forming interdisciplinary fields and how language reflects (and is reflected by) these interactions. Specifically, bibliographical citations are investigated in corpora of research articles from three interdisciplines: Educational Neuroscience, Economic History, and Science and Technology Studies, as well as the single-domain disciplines from which they are derived. Comparisons are carried out between the interdisciplinary fields and between those fields and their related single-domain disciplines. The study combines analysis of quantitative data and qualitative interpretation by means of close reading. It concludes that bibliographical citations constitute a viable tool to explore interdisciplinary writing in the fields explored. The Element demonstrates that it is possible to describe epistemologically distinct types of interdisciplinarity by means of linguistic evidence.