This chronological overview of the development of interdisciplinary theory starts with the pre-cursors of theory: the development and elaboration of the definition of interdisciplinary studies, influential but problematic images of interdisciplinary studies proposed by Donald Campbell and Erich Jantsch, and best practices in interdisciplinary studies identified through AIS. It continues with a review of the motivation behind a theory of interdisciplinary studies, the controversy it engendered, and an assessment of what remains to be done. It concludes with the challenges to theory posed by new types of interdisciplinary studies, especially transdisciplinary studies and the science of team science.