The complexity of interdisciplinary research, including the time needed to understand multiple approaches and develop skills, within a university structure organised in disciplines, means that interdisciplinary research can be difficult for a developing researcher. However, early- and mid-career researchers (EMCRs) are key to the future of interdisciplinary research. This paper asks, 'Why and how do EMCRs become interdisciplinarians?' It draws on analysis of interviews supplemented by ethnographic observations from a research project exploring the development of interdisciplinary expertise in universities. This paper outlines six common intersecting career pathways through which EMCRs come to work across disciplines. The diverse and often ad hoc nature of interdisciplinary research careers implies that support systems, resources and training need to be adaptive and flexible.